diff --git a/dev/src/expression.ts b/dev/src/expression.ts
index 399433096..c0f87daf5 100644
--- a/dev/src/expression.ts
+++ b/dev/src/expression.ts
@@ -1942,38 +1942,6 @@ export class Field extends Expr implements Selectable {
   }
 }
 
-/**
- * @beta
- */
-export class Fields extends Expr implements Selectable {
-  exprType: ExprType = 'Field';
-  selectable = true as const;
-
-  private constructor(private fields: Field[]) {
-    super();
-  }
-
-  static of(name: string, ...others: string[]): Fields {
-    return new Fields([Field.of(name), ...others.map(Field.of)]);
-  }
-
-  static ofAll(): Fields {
-    return new Fields([]);
-  }
-
-  fieldList(): Field[] {
-    return this.fields.map(f => f);
-  }
-
-  _toProto(serializer: Serializer): api.IValue {
-    return {
-      arrayValue: {
-        values: this.fields.map(f => f._toProto(serializer)),
-      },
-    };
-  }
-}
-
 /**
  * @beta
  *
diff --git a/dev/src/index.ts b/dev/src/index.ts
index 5b83d596c..387ff08df 100644
--- a/dev/src/index.ts
+++ b/dev/src/index.ts
@@ -137,7 +137,6 @@ export {
   Expr,
   ExprWithAlias,
   Field,
-  Fields,
   Constant,
   Function,
   Ordering,
diff --git a/dev/src/pipeline-util.ts b/dev/src/pipeline-util.ts
index 1bb77b482..3200eba5d 100644
--- a/dev/src/pipeline-util.ts
+++ b/dev/src/pipeline-util.ts
@@ -156,7 +156,6 @@ export class ExecutionUtil<AppModelType> {
         enc,
         callback
       ) => {
-        console.log(`Pipeline response: ${JSON.stringify(proto, null, 2)}`);
         if (proto === NOOP_MESSAGE) {
           callback(undefined);
           return;
@@ -220,10 +219,6 @@ export class ExecutionUtil<AppModelType> {
           explainOptions
         );
 
-        console.log(
-          `Executing pipeline: \n ${JSON.stringify(request, null, 2)}`
-        );
-
         let streamActive: Deferred<boolean>;
         do {
           streamActive = new Deferred<boolean>();
diff --git a/dev/src/pipeline.ts b/dev/src/pipeline.ts
index 4fc32d402..c3851558d 100644
--- a/dev/src/pipeline.ts
+++ b/dev/src/pipeline.ts
@@ -21,7 +21,6 @@ import {
   Expr,
   ExprWithAlias,
   Field,
-  Fields,
   FilterCondition,
   Function,
   Ordering,
@@ -51,6 +50,12 @@ import {
   Stage,
   Distinct,
   RemoveFields,
+  Replace,
+  Sample,
+  SampleOptions,
+  Union,
+  Unnest,
+  UnnestOptions,
 } from './stage';
 import {ApiMapValue, defaultPipelineConverter} from './types';
 import * as protos from '../protos/firestore_v1_proto_api';
@@ -107,21 +112,21 @@ export class PipelineSource implements firestore.PipelineSource {
  *
  * // Example 1: Select specific fields and rename 'rating' to 'bookRating'
  * const results1 = await db.pipeline()
- *     .collection("books")
- *     .select("title", "author", Field.of("rating").as("bookRating"))
+ *     .collection('books')
+ *     .select('title', 'author', Field.of('rating').as('bookRating'))
  *     .execute();
  *
- * // Example 2: Filter documents where 'genre' is "Science Fiction" and 'published' is after 1950
+ * // Example 2: Filter documents where 'genre' is 'Science Fiction' and 'published' is after 1950
  * const results2 = await db.pipeline()
- *     .collection("books")
- *     .where(and(Field.of("genre").eq("Science Fiction"), Field.of("published").gt(1950)))
+ *     .collection('books')
+ *     .where(and(Field.of('genre').eq('Science Fiction'), Field.of('published').gt(1950)))
  *     .execute();
  *
  * // Example 3: Calculate the average rating of books published after 1980
  * const results3 = await db.pipeline()
- *     .collection("books")
- *     .where(Field.of("published").gt(1980))
- *     .aggregate(avg(Field.of("rating")).as("averageRating"))
+ *     .collection('books')
+ *     .where(Field.of('published').gt(1980))
+ *     .aggregate(avg(Field.of('rating')).as('averageRating'))
  *     .execute();
  * ```
  */
@@ -134,6 +139,12 @@ export class Pipeline<AppModelType = firestore.DocumentData>
     private converter: firestore.FirestorePipelineConverter<AppModelType> = defaultPipelineConverter()
   ) {}
 
+  private _addStage(stage: Stage): Pipeline<AppModelType> {
+    const copy = this.stages.map(s => s);
+    copy.push(stage);
+    return new Pipeline(this.db, copy, this.converter);
+  }
+
   /**
    * Adds new fields to outputs from previous stages.
    *
@@ -150,10 +161,10 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * Example:
    *
    * ```typescript
-   * firestore.pipeline().collection("books")
+   * firestore.pipeline().collection('books')
    *   .addFields(
-   *     Field.of("rating").as("bookRating"), // Rename 'rating' to 'bookRating'
-   *     add(5, Field.of("quantity")).as("totalCost")  // Calculate 'totalCost'
+   *     Field.of('rating').as('bookRating'), // Rename 'rating' to 'bookRating'
+   *     add(5, Field.of('quantity')).as('totalCost')  // Calculate 'totalCost'
    *   );
    * ```
    *
@@ -161,9 +172,7 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * @return A new Pipeline object with this stage appended to the stage list.
    */
   addFields(...fields: firestore.Selectable[]): Pipeline<AppModelType> {
-    const copy = this.stages.map(s => s);
-    copy.push(new AddFields(this.selectablesToMap(fields)));
-    return new Pipeline(this.db, copy, this.converter);
+    return this._addStage(new AddFields(this.selectablesToMap(fields)));
   }
 
   /**
@@ -172,11 +181,11 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * Example:
    *
    * ```typescript
-   * firestore.pipeline().collection("books")
+   * firestore.pipeline().collection('books')
    *   // removes field 'rating' and 'cost' from the previous stage outputs.
    *   .removeFields(
-   *     Field.of("rating"),
-   *     "cost"
+   *     Field.of('rating'),
+   *     'cost'
    *   );
    * ```
    *
@@ -186,13 +195,11 @@ export class Pipeline<AppModelType = firestore.DocumentData>
   removeFields(
     ...fields: (firestore.Field | string)[]
   ): Pipeline<AppModelType> {
-    const copy = this.stages.map(s => s);
-    copy.push(
+    return this._addStage(
       new RemoveFields(
         fields.map(f => (typeof f === 'string' ? Field.of(f) : (f as Field)))
       )
     );
-    return new Pipeline(this.db, copy, this.converter);
   }
 
   /**
@@ -214,11 +221,11 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * <p>Example:
    *
    * ```typescript
-   * firestore.pipeline().collection("books")
+   * firestore.pipeline().collection('books')
    *   .select(
-   *     "firstName",
-   *     Field.of("lastName"),
-   *     Field.of("address").toUppercase().as("upperAddress"),
+   *     'firstName',
+   *     Field.of('lastName'),
+   *     Field.of('address').toUppercase().as('upperAddress'),
    *   );
    * ```
    *
@@ -229,9 +236,7 @@ export class Pipeline<AppModelType = firestore.DocumentData>
   select(
     ...selections: (firestore.Selectable | string)[]
   ): Pipeline<AppModelType> {
-    const copy = this.stages.map(s => s);
-    copy.push(new Select(this.selectablesToMap(selections)));
-    return new Pipeline(this.db, copy, this.converter);
+    return this._addStage(new Select(this.selectablesToMap(selections)));
   }
 
   private selectablesToMap(
@@ -243,14 +248,11 @@ export class Pipeline<AppModelType = firestore.DocumentData>
         result.set(selectable as string, Field.of(selectable));
       } else if (selectable instanceof Field) {
         result.set((selectable as Field).fieldName(), selectable);
-      } else if (selectable instanceof Fields) {
-        const fields = selectable as Fields;
-        for (const field of fields.fieldList()) {
-          result.set(field.fieldName(), field);
-        }
       } else if (selectable instanceof ExprWithAlias) {
         const expr = selectable as ExprWithAlias<Expr>;
         result.set(expr.alias, expr.expr);
+      } else {
+        throw new Error('unexpected selectable: ' + selectable);
       }
     }
     return result;
@@ -260,7 +262,7 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * Filters the documents from previous stages to only include those matching the specified {@link
    * FilterCondition}.
    *
-   * <p>This stage allows you to apply conditions to the data, similar to a "WHERE" clause in SQL.
+   * <p>This stage allows you to apply conditions to the data, similar to a 'WHERE' clause in SQL.
    * You can filter documents based on their field values, using implementations of {@link
    * FilterCondition}, typically including but not limited to:
    *
@@ -275,11 +277,11 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * <p>Example:
    *
    * ```typescript
-   * firestore.pipeline().collection("books")
+   * firestore.pipeline().collection('books')
    *   .where(
    *     and(
-   *         gt(Field.of("rating"), 4.0),   // Filter for ratings greater than 4.0
-   *         Field.of("genre").eq("Science Fiction") // Equivalent to gt("genre", "Science Fiction")
+   *         gt(Field.of('rating'), 4.0),   // Filter for ratings greater than 4.0
+   *         Field.of('genre').eq('Science Fiction') // Equivalent to gt('genre', 'Science Fiction')
    *     )
    *   );
    * ```
@@ -288,9 +290,7 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * @return A new Pipeline object with this stage appended to the stage list.
    */
   where(condition: FilterCondition & firestore.Expr): Pipeline<AppModelType> {
-    const copy = this.stages.map(s => s);
-    copy.push(new Where(condition));
-    return new Pipeline(this.db, copy, this.converter);
+    return this._addStage(new Where(condition));
   }
 
   /**
@@ -304,8 +304,8 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    *
    * ```typescript
    * // Retrieve the second page of 20 results
-   * firestore.pipeline().collection("books")
-   *     .sort(Field.of("published").descending())
+   * firestore.pipeline().collection('books')
+   *     .sort(Field.of('published').descending())
    *     .offset(20)  // Skip the first 20 results
    *     .limit(20);   // Take the next 20 results
    * ```
@@ -314,9 +314,7 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * @return A new Pipeline object with this stage appended to the stage list.
    */
   offset(offset: number): Pipeline<AppModelType> {
-    const copy = this.stages.map(s => s);
-    copy.push(new Offset(offset));
-    return new Pipeline(this.db, copy, this.converter);
+    return this._addStage(new Offset(offset));
   }
 
   /**
@@ -336,8 +334,8 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    *
    * ```typescript
    * // Limit the results to the top 10 highest-rated books
-   * firestore.pipeline().collection("books")
-   *     .sort(Field.of("rating").descending())
+   * firestore.pipeline().collection('books')
+   *     .sort(Field.of('rating').descending())
    *     .limit(10);
    * ```
    *
@@ -345,9 +343,7 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * @return A new Pipeline object with this stage appended to the stage list.
    */
   limit(limit: number): Pipeline<AppModelType> {
-    const copy = this.stages.map(s => s);
-    copy.push(new Limit(limit));
-    return new Pipeline(this.db, copy, this.converter);
+    return this._addStage(new Limit(limit));
   }
 
   /**
@@ -369,21 +365,19 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    *
    * ```typescript
    * // Get a list of unique author names in uppercase and genre combinations.
-   * firestore.pipeline().collection("books")
-   *     .distinct(toUppercase(Field.of("author")).as("authorName"), Field.of("genre"), "publishedAt")
-   *     .select("authorName");
+   * firestore.pipeline().collection('books')
+   *     .distinct(toUppercase(Field.of('author')).as('authorName'), Field.of('genre'), 'publishedAt')
+   *     .select('authorName');
    * ```
    *
-   * @param selectables The {@link Selectable} expressions to consider when determining distinct
+   * @param groups The {@link Selectable} expressions to consider when determining distinct
    *     value combinations or {@code string}s representing field names.
    * @return A new {@code Pipeline} object with this stage appended to the stage list.
    */
   distinct(
     ...groups: (string | firestore.Selectable)[]
   ): Pipeline<AppModelType> {
-    const copy = this.stages.map(s => s);
-    copy.push(new Distinct(this.selectablesToMap(groups || [])));
-    return new Pipeline(this.db, copy, this.converter);
+    return this._addStage(new Distinct(this.selectablesToMap(groups || [])));
   }
 
   /**
@@ -397,10 +391,10 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    *
    * ```typescript
    * // Calculate the average rating and the total number of books
-   * firestore.pipeline().collection("books")
+   * firestore.pipeline().collection('books')
    *     .aggregate(
-   *         Field.of("rating").avg().as("averageRating"),
-   *         countAll().as("totalBooks")
+   *         Field.of('rating').avg().as('averageRating'),
+   *         countAll().as('totalBooks')
    *     );
    * ```
    *
@@ -432,10 +426,10 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    *
    * ```typescript
    * // Calculate the average rating for each genre.
-   * firestore.pipeline().collection("books")
+   * firestore.pipeline().collection('books')
    *   .aggregate({
-   *       accumulators: [avg(Field.of("rating")).as("avg_rating")]
-   *       groups: ["genre"]
+   *       accumulators: [avg(Field.of('rating')).as('avg_rating')],
+   *       groups: ['genre']
    *       });
    * ```
    *
@@ -456,9 +450,8 @@ export class Pipeline<AppModelType = firestore.DocumentData>
         },
     ...rest: firestore.AccumulatorTarget[]
   ): Pipeline<AppModelType> {
-    const copy = this.stages.map(s => s);
     if ('accumulators' in optionsOrTarget) {
-      copy.push(
+      return this._addStage(
         new Aggregate(
           new Map<string, Accumulator>(
             optionsOrTarget.accumulators.map(
@@ -472,7 +465,7 @@ export class Pipeline<AppModelType = firestore.DocumentData>
         )
       );
     } else {
-      copy.push(
+      return this._addStage(
         new Aggregate(
           new Map<string, Accumulator>(
             [optionsOrTarget, ...rest].map(target => [
@@ -484,14 +477,226 @@ export class Pipeline<AppModelType = firestore.DocumentData>
         )
       );
     }
-    return new Pipeline(this.db, copy, this.converter);
   }
 
-  findNearest(options: firestore.FindNearestOptions): Pipeline<AppModelType>;
   findNearest(options: firestore.FindNearestOptions): Pipeline<AppModelType> {
-    const copy = this.stages.map(s => s);
-    copy.push(new FindNearest(options));
-    return new Pipeline(this.db, copy);
+    return this._addStage(new FindNearest(options));
+  }
+
+  /**
+   * Fully overwrites all fields in a document with those coming from a nested map.
+   *
+   * <p>This stage allows you to emit a map value as a document. Each key of the map becomes a field
+   * on the document that contains the corresponding value.
+   *
+   * <p>Example:
+   *
+   * <pre>{@code
+   * // Input.
+   * // {
+   * //  'name': 'John Doe Jr.',
+   * //  'parents': {
+   * //    'father': 'John Doe Sr.',
+   * //    'mother': 'Jane Doe'
+   * // }
+   *
+   * // Emit parents as document.
+   * firestore.pipeline().collection('people').replace(Field.of('parents'));
+   *
+   * // Output
+   * // {
+   * //  'father': 'John Doe Sr.',
+   * //  'mother': 'Jane Doe'
+   * // }
+   * }</pre>
+   *
+   * @param field The {@link Selectable} field containing the nested map.
+   * @return A new {@code Pipeline} object with this stage appended to the stage list.
+   */
+  replace(
+    field: firestore.Selectable | string
+  ): FirebaseFirestore.Pipeline<AppModelType> {
+    return this._addStage(
+      new Replace(this.selectableToExpr(field), 'full_replace')
+    );
+  }
+
+  private selectableToExpr(
+    selectable: FirebaseFirestore.Selectable | string
+  ): Expr {
+    if (typeof selectable === 'string') {
+      return Field.of(selectable);
+    } else if (selectable instanceof Field) {
+      return selectable;
+    } else if (selectable instanceof ExprWithAlias) {
+      return selectable.expr;
+    } else {
+      throw new Error('unexpected selectable: ' + selectable);
+    }
+  }
+
+  /**
+   * Performs a pseudo-random sampling of the documents from the previous stage.
+   *
+   * <p>This stage will filter documents pseudo-randomly. The parameter specifies how number of
+   * documents to be returned.
+   *
+   * <p>Examples:
+   *
+   * <pre>{@code
+   * // Sample 25 books, if available.
+   * firestore.pipeline().collection('books')
+   *     .sample(25);
+   * }
+   * </pre>
+   *
+   * @param documents The number of documents to sample..
+   * @return A new {@code Pipeline} object with this stage appended to the stage list.
+   */
+  sample(documents: number): FirebaseFirestore.Pipeline<AppModelType>;
+
+  /**
+   * Performs a pseudo-random sampling of the documents from the previous stage.
+   *
+   * <p>This stage will filter documents pseudo-randomly. The 'options' parameter specifies how
+   * sampling will be performed. See {@code SampleOptions} for more information.
+   *
+   * <p>Examples:
+   *
+   * // Sample 10 books, if available.
+   * firestore.pipeline().collection("books")
+   *     .sample({ documents: 10 });
+   *
+   * // Sample 50% of books.
+   * firestore.pipeline().collection("books")
+   *     .sample({ percentage: 0.5 });
+   * }
+   * </pre>
+   *
+   * @param options The {@code SampleOptions} specifies how sampling is performed.
+   * @return A new {@code Pipeline} object with this stage appended to the stage list.
+   */
+  sample(
+    options: {percentage: number} | {documents: number}
+  ): FirebaseFirestore.Pipeline<AppModelType>;
+  sample(
+    documentsOrOptions: number | {percentage: number} | {documents: number}
+  ): FirebaseFirestore.Pipeline<AppModelType> {
+    if (typeof documentsOrOptions === 'number') {
+      return this._addStage(
+        new Sample({limit: documentsOrOptions, mode: 'documents'})
+      );
+    } else if ('percentage' in documentsOrOptions) {
+      return this._addStage(
+        new Sample({limit: documentsOrOptions.percentage, mode: 'percent'})
+      );
+    } else {
+      return this._addStage(
+        new Sample({limit: documentsOrOptions.documents, mode: 'documents'})
+      );
+    }
+  }
+
+  /**
+   * Performs union of all documents from two pipelines, including duplicates.
+   *
+   * <p>This stage will pass through documents from previous stage, and also pass through documents
+   * from previous stage of the `other` {@code Pipeline} given in parameter. The order of documents
+   * emitted from this stage is undefined.
+   *
+   * <p>Example:
+   *
+   * <pre>{@code
+   * // Emit documents from books collection and magazines collection.
+   * firestore.pipeline().collection('books')
+   *     .union(firestore.pipeline().collection('magazines'));
+   * }</pre>
+   *
+   * @param other The other {@code Pipeline} that is part of union.
+   * @return A new {@code Pipeline} object with this stage appended to the stage list.
+   */
+  union(
+    other: FirebaseFirestore.Pipeline<AppModelType>
+  ): FirebaseFirestore.Pipeline<AppModelType> {
+    return this._addStage(new Union(other));
+  }
+
+  /**
+   * Produces a document for each element in array found in previous stage document.
+   *
+   * <p>For each previous stage document, this stage will emit zero or more augmented documents. The
+   * input array found in the previous stage document field specified by the `fieldName` parameter,
+   * will for each input array element produce an augmented document. The input array element will
+   * augment the previous stage document by replacing the field specified by `fieldName` parameter
+   * with the element value.
+   *
+   * <p>In other words, the field containing the input array will be removed from the augmented
+   * document and replaced by the corresponding array element.
+   *
+   * <p>Example:
+   *
+   * <pre>{@code
+   * // Input:
+   * // { 'title': 'The Hitchhiker's Guide to the Galaxy', 'tags': [ 'comedy', 'space', 'adventure' ], ... }
+   *
+   * // Emit a book document for each tag of the book.
+   * firestore.pipeline().collection('books')
+   *     .unnest('tags');
+   *
+   * // Output:
+   * // { 'title': 'The Hitchhiker's Guide to the Galaxy', 'tags': 'comedy', ... }
+   * // { 'title': 'The Hitchhiker's Guide to the Galaxy', 'tags': 'space', ... }
+   * // { 'title': 'The Hitchhiker's Guide to the Galaxy', 'tags': 'adventure', ... }
+   * }</pre>
+   *
+   * @param field The name of the field containing the array.
+   * @return A new {@code Pipeline} object with this stage appended to the stage list.
+   */
+  unnest(
+    field: FirebaseFirestore.Selectable | string
+  ): FirebaseFirestore.Pipeline<AppModelType>;
+
+  /**
+   * Produces a document for each element in array found in previous stage document.
+   *
+   * <p>For each previous stage document, this stage will emit zero or more augmented documents. The
+   * input array found in the previous stage document field specified by the `fieldName` parameter,
+   * will for each input array element produce an augmented document. The input array element will
+   * augment the previous stage document by replacing the field specified by `fieldName` parameter
+   * with the element value.
+   *
+   * <p>In other words, the field containing the input array will be removed from the augmented
+   * document and replaced by the corresponding array element.
+   *
+   * <p>Example:
+   *
+   * <pre>{@code
+   * // Input:
+   * // { 'title': 'The Hitchhiker's Guide to the Galaxy', 'tags': [ 'comedy', 'space', 'adventure' ], ... }
+   *
+   * // Emit a book document for each tag of the book.
+   * firestore.pipeline().collection('books')
+   *     .unnest({ field: 'tags', UnnestOptions.indexField('tagIndex'));
+   *
+   * // Output:
+   * // { 'title': 'The Hitchhiker's Guide to the Galaxy', 'tagIndex': 0, 'tags': 'comedy', ... }
+   * // { 'title': 'The Hitchhiker's Guide to the Galaxy', 'tagIndex': 1, 'tags': 'space', ... }
+   * // { 'title': 'The Hitchhiker's Guide to the Galaxy', 'tagIndex': 2, 'tags': 'adventure', ... }
+   * }</pre>
+   *
+   * @param fieldName The name of the field containing the array.
+   * @param options The {@code UnnestOptions} options.
+   * @return A new {@code Pipeline} object with this stage appended to the stage list.
+   */
+  unnest(options: UnnestOptions): FirebaseFirestore.Pipeline<AppModelType>;
+  unnest(
+    fieldOrOptions: FirebaseFirestore.Selectable | string | UnnestOptions
+  ): FirebaseFirestore.Pipeline<AppModelType> {
+    if (typeof fieldOrOptions === 'string' || 'selectable' in fieldOrOptions) {
+      return this._addStage(new Unnest({field: fieldOrOptions}));
+    } else {
+      return this._addStage(new Unnest(fieldOrOptions));
+    }
   }
 
   /**
@@ -508,10 +713,10 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * ```typescript
    * // Sort books by rating in descending order, and then by title in ascending order for books
    * // with the same rating
-   * firestore.pipeline().collection("books")
+   * firestore.pipeline().collection('books')
    *     .sort(
-   *         Field.of("rating").descending(),
-   *         Field.of("title").ascending()
+   *         Field.of('rating').descending(),
+   *         Field.of('title').ascending()
    *     );
    * ```
    *
@@ -528,16 +733,13 @@ export class Pipeline<AppModelType = firestore.DocumentData>
         },
     ...rest: Ordering[]
   ): Pipeline<AppModelType> {
-    const copy = this.stages.map(s => s);
     // Option object
     if ('orderings' in optionsOrOrderings) {
-      copy.push(new Sort(optionsOrOrderings.orderings));
+      return this._addStage(new Sort(optionsOrOrderings.orderings));
     } else {
       // Ordering object
-      copy.push(new Sort([optionsOrOrderings, ...rest]));
+      return this._addStage(new Sort([optionsOrOrderings, ...rest]));
     }
-
-    return new Pipeline(this.db, copy, this.converter);
   }
 
   /**
@@ -547,13 +749,13 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * stages. Each generic stage is defined by a unique `name` and a set of `params` that control its
    * behavior.
    *
-   * <p>Example (Assuming there is no "where" stage available in SDK):
+   * <p>Example (Assuming there is no 'where' stage available in SDK):
    *
    * ```typescript
-   * // Assume we don't have a built-in "where" stage
-   * firestore.pipeline().collection("books")
-   *     .genericStage("where", [Field.of("published").lt(1900)]) // Custom "where" stage
-   *     .select("title", "author");
+   * // Assume we don't have a built-in 'where' stage
+   * firestore.pipeline().collection('books')
+   *     .genericStage('where', [Field.of('published').lt(1900)]) // Custom 'where' stage
+   *     .select('title', 'author');
    * ```
    *
    * @param name The unique name of the generic stage to add.
@@ -561,9 +763,7 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * @return A new {@code Pipeline} object with this stage appended to the stage list.
    */
   genericStage(name: string, params: any[]): Pipeline<AppModelType> {
-    const copy = this.stages.map(s => s);
-    copy.push(new GenericStage(name, params));
-    return new Pipeline(this.db, copy, this.converter);
+    return this._addStage(new GenericStage(name, params));
   }
 
   withConverter(converter: null): Pipeline;
@@ -654,9 +854,9 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    * <p>Example:
    *
    * ```typescript
-   * const futureResults = await firestore.pipeline().collection("books")
-   *     .where(gt(Field.of("rating"), 4.5))
-   *     .select("title", "author", "rating")
+   * const futureResults = await firestore.pipeline().collection('books')
+   *     .where(gt(Field.of('rating'), 4.5))
+   *     .select('title', 'author', 'rating')
    *     .execute();
    * ```
    *
@@ -688,9 +888,9 @@ export class Pipeline<AppModelType = firestore.DocumentData>
    *
    * @example
    * ```typescript
-   * firestore.pipeline().collection("books")
-   *     .where(gt(Field.of("rating"), 4.5))
-   *     .select("title", "author", "rating")
+   * firestore.pipeline().collection('books')
+   *     .where(gt(Field.of('rating'), 4.5))
+   *     .select('title', 'author', 'rating')
    *     .stream()
    *     .on('data', (pipelineResult) => {})
    *     .on('end', () => {});
diff --git a/dev/src/serializer.ts b/dev/src/serializer.ts
index a66bba1b4..bbb703e5a 100644
--- a/dev/src/serializer.ts
+++ b/dev/src/serializer.ts
@@ -105,6 +105,21 @@ export class Serializer {
    * @param val The object to encode
    * @returns The Firestore Proto or null if we are deleting a field.
    */
+  encodeValue(val: FieldTransform | undefined): null;
+  encodeValue(
+    val:
+      | string
+      | boolean
+      | number
+      | bigint
+      | Date
+      | null
+      | Buffer
+      | Uint8Array
+      | VectorValue
+      | Map<string, unknown>
+  ): api.IValue;
+  encodeValue(val: unknown): api.IValue | null;
   encodeValue(val: unknown): api.IValue | null {
     if (val instanceof FieldTransform) {
       return null;
diff --git a/dev/src/stage.ts b/dev/src/stage.ts
index 5be9be513..8ed76400e 100644
--- a/dev/src/stage.ts
+++ b/dev/src/stage.ts
@@ -196,7 +196,7 @@ export class Where implements Stage {
  * @beta
  */
 export interface FindNearestOptions {
-  field: firestore.Field;
+  field: firestore.Field | string;
   vectorValue: firestore.VectorValue | number[];
   distanceMeasure: 'euclidean' | 'cosine' | 'dot_product';
   limit?: number;
@@ -224,7 +224,10 @@ export class FindNearest implements Stage {
     return {
       name: this.name,
       args: [
-        (this._options.field as unknown as Field)._toProto(serializer),
+        (typeof this._options.field === 'string'
+          ? Field.of(this._options.field)
+          : (this._options.field as unknown as Field)
+        )._toProto(serializer),
         this._options.vectorValue instanceof VectorValue
           ? serializer.encodeValue(this._options.vectorValue)!
           : serializer.encodeVector(this._options.vectorValue as number[]),
@@ -235,6 +238,78 @@ export class FindNearest implements Stage {
   }
 }
 
+/**
+ * @beta
+ */
+export interface SampleOptions {
+  limit: number;
+  mode: 'documents' | 'percent';
+}
+
+/**
+ * @beta
+ */
+export class Sample implements Stage {
+  name = 'sample';
+
+  constructor(private _options: SampleOptions) {}
+
+  _toProto(serializer: Serializer): api.Pipeline.IStage {
+    return {
+      name: this.name,
+      args: [
+        serializer.encodeValue(this._options.limit)!,
+        serializer.encodeValue(this._options.mode)!,
+      ],
+    };
+  }
+}
+
+/**
+ * @beta
+ */
+export class Union implements Stage {
+  name = 'union';
+
+  constructor(private _other: FirebaseFirestore.Pipeline<unknown>) {}
+
+  _toProto(serializer: Serializer): api.Pipeline.IStage {
+    return {
+      name: this.name,
+      args: [serializer.encodeValue(this._other)!],
+    };
+  }
+}
+
+/**
+ * @beta
+ */
+export interface UnnestOptions {
+  field: firestore.Selectable | string;
+  indexField?: string;
+}
+
+/**
+ * @beta
+ */
+export class Unnest implements Stage {
+  name = 'unnest';
+
+  constructor(private options: UnnestOptions) {}
+
+  _toProto(serializer: Serializer): api.Pipeline.IStage {
+    const args = [serializer.encodeValue(this.options.field)!];
+    const indexField = this.options.indexField;
+    if (indexField) {
+      args.push(serializer.encodeValue(indexField));
+    }
+    return {
+      name: this.name,
+      args: args,
+    };
+  }
+}
+
 /**
  * @beta
  */
@@ -267,6 +342,31 @@ export class Offset implements Stage {
   }
 }
 
+/**
+ * @beta
+ */
+export class Replace implements Stage {
+  name = 'replace';
+
+  constructor(
+    private field: Expr,
+    private mode:
+      | 'full_replace'
+      | 'merge_prefer_nest'
+      | 'merge_prefer_parent' = 'full_replace'
+  ) {}
+
+  _toProto(serializer: Serializer): api.Pipeline.IStage {
+    return {
+      name: this.name,
+      args: [
+        serializer.encodeValue(this.field)!,
+        serializer.encodeValue(this.mode),
+      ],
+    };
+  }
+}
+
 /**
  * @beta
  */
diff --git a/dev/system-test/firestore.ts b/dev/system-test/firestore.ts
index cf040220b..7181d9fdb 100644
--- a/dev/system-test/firestore.ts
+++ b/dev/system-test/firestore.ts
@@ -19,7 +19,6 @@ import {
   QuerySnapshot,
   SetOptions,
   Settings,
-  VectorValue,
   WithFieldValue,
 } from '@google-cloud/firestore';
 
diff --git a/dev/system-test/pipeline.ts b/dev/system-test/pipeline.ts
index 317c4e839..1d0181d81 100644
--- a/dev/system-test/pipeline.ts
+++ b/dev/system-test/pipeline.ts
@@ -64,7 +64,7 @@ import {PipelineResult} from '../src/pipeline';
 import {verifyInstance} from '../test/util/helpers';
 import {DeferredPromise, getTestRoot} from './firestore';
 
-describe.only('Pipeline class', () => {
+describe('Pipeline class', () => {
   let firestore: Firestore;
   let randomCol: CollectionReference;
 
@@ -972,4 +972,39 @@ describe.only('Pipeline class', () => {
       .execute();
     expectResults(result, {title: "The Hitchhiker's Guide to the Galaxy"});
   });
+
+  it('run pipeline with sample limit of 3', async () => {
+    const results = await randomCol.pipeline().sample(3).execute();
+    expect(results.length).to.equal(3);
+  });
+
+  it('run pipeline with sample limit of {documents: 3}', async () => {
+    const results = await randomCol.pipeline().sample({documents: 3}).execute();
+    expect(results.length).to.equal(3);
+  });
+
+  it('run pipeline with sample limit of {percentage: 0.6}', async () => {
+    const results = await randomCol
+      .pipeline()
+      .sample({percentage: 0.6})
+      .execute();
+    expect(results.length).to.equal(6);
+  });
+
+  it('run pipeline with union', async () => {
+    const results = await randomCol
+      .pipeline()
+      .union(randomCol.pipeline())
+      .execute();
+    expect(results.length).to.equal(20);
+  });
+
+  it('run pipeline with unnest', async () => {
+    const results = await randomCol
+      .pipeline()
+      .where(eq('title', "The Hitchhiker's Guide to the Galaxy"))
+      .unnest('tags')
+      .execute();
+    expect(results.length).to.equal(3);
+  });
 });
diff --git a/dev/system-test/query.ts b/dev/system-test/query.ts
index 84853f79b..e7d800382 100644
--- a/dev/system-test/query.ts
+++ b/dev/system-test/query.ts
@@ -1803,8 +1803,8 @@ describe('Query class', () => {
   });
 
   (process.env.FIRESTORE_EMULATOR_HOST === undefined
-    ? describe
-    : describe.only)('multiple inequality', () => {
+    ? describe.skip
+    : describe)('multiple inequality', () => {
     it('supports multiple inequality queries', async () => {
       const collection = await testCollectionWithDocs({
         doc1: {key: 'a', sort: 0, v: 0},
diff --git a/types/firestore.d.ts b/types/firestore.d.ts
index 152f35996..83fab89c2 100644
--- a/types/firestore.d.ts
+++ b/types/firestore.d.ts
@@ -4733,22 +4733,6 @@ declare namespace FirebaseFirestore {
     fieldName(): string;
   }
 
-  /**
-   * @beta
-   */
-  export class Fields extends Expr implements Selectable {
-    exprType: ExprType;
-    selectable: true;
-    static of(name: string, ...others: string[]): Fields;
-    static ofAll(): Fields;
-    /**
-     * Returns the list of fields.
-     *
-     * @return The list of fields.
-     */
-    fieldList(): Field[];
-  }
-
   /**
    * @beta
    *
@@ -5034,6 +5018,13 @@ declare namespace FirebaseFirestore {
     filterable: true;
   }
 
+  /**
+   * @beta
+   */
+  export class NotEqAny extends Function implements FilterCondition {
+    filterable: true;
+  }
+
   /**
    * @beta
    */
@@ -8832,7 +8823,7 @@ declare namespace FirebaseFirestore {
    * @beta
    */
   export interface FindNearestOptions {
-    field: Field;
+    field: Field | string;
     vectorValue: VectorValue | number[];
     distanceMeasure: 'euclidean' | 'cosine' | 'dot_product';
     limit?: number;
@@ -8860,6 +8851,28 @@ declare namespace FirebaseFirestore {
     name: string;
   }
 
+  /**
+   * @beta
+   */
+  export class Replace implements Stage {
+    name: string;
+  }
+
+  /**
+   * @beta
+   */
+  export class Sample implements Stage {
+    name: string;
+  }
+
+  /**
+   * @beta
+   */
+  export interface SampleOptions {
+    limit: number;
+    mode: 'documents' | 'percent';
+  }
+
   /**
    * @beta
    */
@@ -8874,6 +8887,28 @@ declare namespace FirebaseFirestore {
     name: string;
   }
 
+  /**
+   * @beta
+   */
+  export class Union implements Stage {
+    name: string;
+  }
+
+  /**
+   * @beta
+   */
+  export class Unnest implements Stage {
+    name: string;
+  }
+
+  /**
+   * @beta
+   */
+  export interface UnnestOptions {
+    field: Selectable | string;
+    indexField?: string;
+  }
+
   /**
    * @beta
    */
@@ -9215,6 +9250,83 @@ declare namespace FirebaseFirestore {
 
     findNearest(options: FindNearestOptions): Pipeline<AppModelType>;
 
+    /**
+     * Fully overwrites all fields in a document with those coming from a nested map.
+     *
+     * <p>This stage allows you to emit a map value as a document. Each key of the map becomes a field
+     * on the document that contains the corresponding value.
+     *
+     * <p>Example:
+     *
+     * <pre>{@code
+     * // Input.
+     * // {
+     * //  "name": "John Doe Jr.",
+     * //  "parents": {
+     * //    "father": "John Doe Sr.",
+     * //    "mother": "Jane Doe"
+     * // }
+     *
+     * // Emit parents as document.
+     * firestore.pipeline().collection("people").replace(Field.of("parents"));
+     *
+     * // Output
+     * // {
+     * //  "father": "John Doe Sr.",
+     * //  "mother": "Jane Doe"
+     * // }
+     * }</pre>
+     *
+     * @param field The {@link Selectable} field containing the nested map.
+     * @return A new {@code Pipeline} object with this stage appended to the stage list.
+     */
+    replace(field: Selectable | string): Pipeline<AppModelType>;
+
+    /**
+     * Performs a pseudo-random sampling of the documents from the previous stage.
+     *
+     * <p>This stage will filter documents pseudo-randomly. The parameter specifies how number of
+     * documents to be returned.
+     *
+     * <p>Examples:
+     *
+     * <pre>{@code
+     * // Sample 25 books, if available.
+     * firestore.pipeline().collection("books")
+     *     .sample(25);
+     * }
+     * </pre>
+     *
+     * @param documents The number of documents to sample.
+     * @return A new {@code Pipeline} object with this stage appended to the stage list.
+     */
+    sample(documents: number): Pipeline<AppModelType>;
+
+    /**
+     * Performs a pseudo-random sampling of the documents from the previous stage.
+     *
+     * <p>This stage will filter documents pseudo-randomly. The 'options' parameter specifies how
+     * sampling will be performed. See {@code SampleOptions} for more information.
+     *
+     * <p>Examples:
+     *
+     * // Sample 10 books, if available.
+     * firestore.pipeline().collection("books")
+     *     .sample({ documents: 10 });
+     *
+     * // Sample 50% of books.
+     * firestore.pipeline().collection("books")
+     *     .sample({ percentage: 0.5 });
+     * }
+     * </pre>
+     *
+     * @param options The {@code SampleOptions} specifies how sampling is performed.
+     * @return A new {@code Pipeline} object with this stage appended to the stage list.
+     */
+    sample(
+      options: {percentage: number} | {documents: number}
+    ): Pipeline<AppModelType>;
+
     /**
      * Sorts the documents from previous stages based on one or more {@link Ordering} criteria.
      *
@@ -9242,6 +9354,59 @@ declare namespace FirebaseFirestore {
     sort(...orderings: Ordering[]): Pipeline<AppModelType>;
     sort(options: {orderings: Ordering[]}): Pipeline<AppModelType>;
 
+    /**
+     * Performs union of all documents from two pipelines, including duplicates.
+     *
+     * <p>This stage will pass through documents from previous stage, and also pass through documents
+     * from previous stage of the `other` {@code Pipeline} given in parameter. The order of documents
+     * emitted from this stage is undefined.
+     *
+     * <p>Example:
+     *
+     * <pre>{@code
+     * // Emit documents from books collection and magazines collection.
+     * firestore.pipeline().collection("books")
+     *     .union(firestore.pipeline().collection("magazines"));
+     * }</pre>
+     *
+     * @param other The other {@code Pipeline} that is part of union.
+     * @return A new {@code Pipeline} object with this stage appended to the stage list.
+     */
+    union(other: Pipeline<AppModelType>): Pipeline<AppModelType>;
+
+    /**
+     * Produces a document for each element in array found in previous stage document.
+     *
+     * <p>For each previous stage document, this stage will emit zero or more augmented documents. The
+     * input array found in the previous stage document field specified by the `fieldName` parameter,
+     * will for each input array element produce an augmented document. The input array element will
+     * augment the previous stage document by replacing the field specified by `fieldName` parameter
+     * with the element value.
+     *
+     * <p>In other words, the field containing the input array will be removed from the augmented
+     * document and replaced by the corresponding array element.
+     *
+     * <p>Example:
+     *
+     * <pre>{@code
+     * // Input:
+     * // { "title": "The Hitchhiker's Guide to the Galaxy", "tags": [ "comedy", "space", "adventure" ], ... }
+     *
+     * // Emit a book document for each tag of the book.
+     * firestore.pipeline().collection("books")
+     *     .unnest("tags");
+     *
+     * // Output:
+     * // { "title": "The Hitchhiker's Guide to the Galaxy", "tags": "comedy", ... }
+     * // { "title": "The Hitchhiker's Guide to the Galaxy", "tags": "space", ... }
+     * // { "title": "The Hitchhiker's Guide to the Galaxy", "tags": "adventure", ... }
+     * }</pre>
+     *
+     * @param field The name of the field containing the array.
+     * @return A new {@code Pipeline} object with this stage appended to the stage list.
+     */
+    unnest(field: Selectable | string): Pipeline<AppModelType>;
+
     /**
      * Adds a generic stage to the pipeline.
      *