From 1918b251d10d0c8ef98624d7b2cd71f594a0f19b Mon Sep 17 00:00:00 2001
From: PascalSenn <senn.pasc@gmail.com>
Date: Thu, 3 Oct 2024 17:30:52 +0200
Subject: [PATCH 1/4] Adds validation rule `@lookup` must not return a list

---
 spec/Section 4 -- Composition.md | 59 ++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/spec/Section 4 -- Composition.md b/spec/Section 4 -- Composition.md
index 35d3b72..6e0d970 100644
--- a/spec/Section 4 -- Composition.md	
+++ b/spec/Section 4 -- Composition.md	
@@ -14,6 +14,65 @@ run in sequence to produce the composite execution schema.
 
 ### Pre Merge Validation
 
+#### `@lookup` must not return a list 
+
+**Error Code**
+
+LOOKUP_MUST_NOT_RETURN_LIST
+
+**Formal Specification**
+
+- Let {fields} be the set of all field definitions annotated with `@lookup` in the schema.
+- For each {field} in {fields}:
+  - Let {type} be the return type of {field}.
+  - {IsListType(type)} must be false.
+
+IsListType(type):
+
+- If {type} is a Non-Null type:
+  - Let {innerType} be the inner type of {type}.
+  - Return {IsSingleObjectType(innerType)}.
+- Else if {type} is a List type:
+  - Return true.
+- Else:
+  - Return false.
+
+**Explanatory Text**
+
+Fields annotated with the `@lookup` directive are intended to retrieve a single entity based on provided arguments. 
+To ensure ambiguity in entity resolution, such fields must return a single object and not a list. 
+This validation rule enforces that any field annotated with `@lookup` must have a return type that is NOT a list.
+
+For example, the following usage is valid:
+
+```graphql example
+extend type Query {
+  userById(id: ID!): User @lookup
+}
+
+type User {
+  id: ID!
+  name: String
+}
+```
+
+In this example, `userById` returns a `User` object, satisfying the requirement.
+
+This counter-example demonstrates an invalid usage:
+
+```graphql counter-example
+extend type Query {
+  usersByRole(role: String!): [User] @lookup
+}
+
+type User {
+  id: ID!
+  name: String
+}
+```
+
+Here, `usersByRole` returns a list of `User` objects, which violates the requirement that a `@lookup` field must return a single object.
+
 ### Merge
 
 ### Post Merge Validation

From 224f9dbb6a8fe6026ab8c792395aac5ee0687c4c Mon Sep 17 00:00:00 2001
From: PascalSenn <senn.pasc@gmail.com>
Date: Thu, 3 Oct 2024 17:51:27 +0200
Subject: [PATCH 2/4] apply fix

Co-authored-by: Glen <glen.84@gmail.com>
---
 spec/Section 4 -- Composition.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/spec/Section 4 -- Composition.md b/spec/Section 4 -- Composition.md
index 6e0d970..dacd61b 100644
--- a/spec/Section 4 -- Composition.md	
+++ b/spec/Section 4 -- Composition.md	
@@ -40,7 +40,7 @@ IsListType(type):
 **Explanatory Text**
 
 Fields annotated with the `@lookup` directive are intended to retrieve a single entity based on provided arguments. 
-To ensure ambiguity in entity resolution, such fields must return a single object and not a list. 
+To avoid ambiguity in entity resolution, such fields must return a single object and not a list. 
 This validation rule enforces that any field annotated with `@lookup` must have a return type that is NOT a list.
 
 For example, the following usage is valid:

From 16ad1fcb11397fa7ac92d6112878a184e85d4c9f Mon Sep 17 00:00:00 2001
From: PascalSenn <senn.pasc@gmail.com>
Date: Thu, 3 Oct 2024 18:51:59 +0200
Subject: [PATCH 3/4] Update Section 4 -- Composition.md

---
 spec/Section 4 -- Composition.md | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/spec/Section 4 -- Composition.md b/spec/Section 4 -- Composition.md
index dacd61b..1870190 100644
--- a/spec/Section 4 -- Composition.md	
+++ b/spec/Section 4 -- Composition.md	
@@ -62,7 +62,7 @@ This counter-example demonstrates an invalid usage:
 
 ```graphql counter-example
 extend type Query {
-  usersByRole(role: String!): [User] @lookup
+  usersByIds(ids: [ID!]!): [User!] @lookup
 }
 
 type User {
@@ -71,7 +71,9 @@ type User {
 }
 ```
 
-Here, `usersByRole` returns a list of `User` objects, which violates the requirement that a `@lookup` field must return a single object.
+Here, `usersByIds` returns a list of `User` objects, which violates the requirement that a `@lookup` field must return a single object.
+
+//TODO explain the `@requires`
 
 ### Merge
 

From 95e32df7b6c1bfa3f34462d52de3ad3a290c7276 Mon Sep 17 00:00:00 2001
From: Michael Staib <michael@chillicream.com>
Date: Thu, 7 Nov 2024 13:19:56 +0100
Subject: [PATCH 4/4] Added Error Code

---
 spec/Section 4 -- Composition.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/spec/Section 4 -- Composition.md b/spec/Section 4 -- Composition.md
index 1870190..005ffe2 100644
--- a/spec/Section 4 -- Composition.md	
+++ b/spec/Section 4 -- Composition.md	
@@ -20,6 +20,8 @@ run in sequence to produce the composite execution schema.
 
 LOOKUP_MUST_NOT_RETURN_LIST
 
+**Severity** ERROR
+
 **Formal Specification**
 
 - Let {fields} be the set of all field definitions annotated with `@lookup` in the schema.