feat: Add support for validating multiple attributes at once

This commit adds support for validating multiple attributes at once with
`validate_presence_of`. This is useful when you want to ensure that
multiple attributes are required.

```ruby
class Example
  include ActiveModel::Model

  attr_accessor :attr1, :attr2

  validates_presence_of :attr1, :attr2
end

RSpec.describe Example do
  it do
    expect(subject).to validate_presence_of(:attr1, :attr2)
  end
end
```

We also add support for using qualifiers with multiple attributes.
There's two caveats: if you use a qualifier, it will apply to all
attributes and only the first failure will be reported.

```ruby
class Example
  include ActiveModel::Model

  attr_accessor :attr1, :attr2

  validates_presence_of :attr1, allow_nil: true
  validates_presence_of :attr2, allow_nil: true
end

RSpec.describe Example do
  it do
    expect(subject).to validate_presence_of(:attr1, :attr2)
  end
end

```

lib/shoulda/matchers/active_model.rb

@@ -1,5 +1,6 @@
 require 'shoulda/matchers/active_model/helpers'
 require 'shoulda/matchers/active_model/qualifiers'
+require 'shoulda/matchers/active_model/matcher_collection'
 require 'shoulda/matchers/active_model/validation_matcher'
 require 'shoulda/matchers/active_model/validation_matcher/build_description'
 require 'shoulda/matchers/active_model/validator'

lib/shoulda/matchers/active_model/allow_value_matcher.rb

@@ -458,7 +458,7 @@ def failure_message
               message << '.'
             else
               message << " producing these validation errors:\n\n"
-              message << validator.all_formatted_validation_error_messages
+              message << validator.formatted_validation_error_messages
             end
           end
 

lib/shoulda/matchers/active_model/helpers.rb

@@ -7,8 +7,10 @@ def pretty_error_messages(object)
           format_validation_errors(object.errors)
         end
 
-        def format_validation_errors(errors)
+        def format_validation_errors(errors, attr = nil)
           list_items = errors.to_hash.keys.map do |attribute|
+            next if attr && attr != attribute
+
             messages = errors[attribute]
             "* #{attribute}: #{messages}"
           end

lib/shoulda/matchers/active_model/matcher_collection.rb

@@ -0,0 +1,59 @@
+module Shoulda
+  module Matchers
+    module ActiveModel
+      # @private
+      class MatcherCollection
+        def initialize(matchers)
+          @matchers = matchers
+        end
+
+        def matches?(subject)
+          @failed_matchers = failed_matchers_for(subject, :matches?)
+          @failed_matchers.empty?
+        end
+
+        def does_not_match?(subject)
+          @failed_matchers = failed_matchers_for(subject, :does_not_match?)
+          @failed_matchers.empty?
+        end
+
+        def failure_message
+          first_failure_message(:failure_message)
+        end
+
+        def failure_message_when_negated
+          first_failure_message(:failure_message_when_negated)
+        end
+
+        def method_missing(method, *args, &block)
+          if all_matchers_respond_to?(method)
+            matchers.each { |matcher| matcher.send(method, *args, &block) }
+            self
+          else
+            super
+          end
+        end
+
+        def respond_to_missing?(method, include_private = false)
+          all_matchers_respond_to?(method) || super
+        end
+
+        private
+
+        attr_reader :matchers
+
+        def failed_matchers_for(subject, method)
+          matchers.reject { |matcher| matcher.send(method, subject) }
+        end
+
+        def first_failure_message(method)
+          @failed_matchers.first&.send(method)
+        end
+
+        def all_matchers_respond_to?(method)
+          matchers.all? { |matcher| matcher.respond_to?(method) }
+        end
+      end
+    end
+  end
+end

lib/shoulda/matchers/active_model/validate_presence_of_matcher.rb

@@ -147,8 +147,10 @@ module ActiveModel
       #
       # @return [ValidatePresenceOfMatcher]
       #
-      def validate_presence_of(attr)
-        ValidatePresenceOfMatcher.new(attr)
+
+      def validate_presence_of(*attrs)
+        matchers = attrs.map { |attr| ValidatePresenceOfMatcher.new(attr) }
+        MatcherCollection.new(matchers)
       end
 
       # @private

lib/shoulda/matchers/active_model/validator.rb

@@ -45,6 +45,10 @@ def validation_exception_message
           validation_result[:validation_exception_message]
         end
 
+        def formatted_validation_error_messages
+          format_validation_errors(all_validation_errors, attribute)
+        end
+
         protected
 
         attr_reader :attribute, :context, :record

spec/unit/shoulda/matchers/active_model/validate_presence_of_matcher_spec.rb

@@ -4,6 +4,69 @@
   include UnitTests::ApplicationConfigurationHelpers
 
   context 'a model with a presence validation' do
+    context 'passing multiple attributes' do
+      it 'accepts' do
+        model = define_model 'Example', attr1: :string, attr2: :string do
+          validates_presence_of(:attr1)
+          validates_presence_of(:attr2)
+        end
+
+        expect(model.new).to validate_presence_of(:attr1, :attr2)
+      end
+
+      it 'fails when used in the negative' do
+        model = define_model 'Example', attr1: :string, attr2: :string do
+          validates_presence_of(:attr1)
+        end
+
+        assertion = lambda do
+          expect(model.new).not_to validate_presence_of(:attr1, :attr2)
+        end
+
+        message = <<-MESSAGE
+Expected Example not to validate that :attr1 cannot be empty/falsy, but
+this could not be proved.
+  After setting :attr1 to ‹nil›, the matcher expected the Example to be
+  valid, but it was invalid instead, producing these validation errors:
+
+  * attr1: ["can't be blank"]
+        MESSAGE
+
+        expect(&assertion).to fail_with_message(message)
+      end
+
+      it 'accepts when using qualifiers' do
+        model = define_model 'Example', attr1: :string, attr2: :string do
+          validates_presence_of(:attr1, allow_nil: true)
+          validates_presence_of(:attr2, allow_nil: true)
+        end
+
+        expect(model.new).to validate_presence_of(:attr1, :attr2).allow_nil
+      end
+
+      it 'rejects when one attribute does not match the qualifier' do
+        model = define_model 'Example', attr1: :string, attr2: :string do
+          validates_presence_of(:attr1, allow_nil: true)
+          validates_presence_of(:attr2)
+        end
+
+        assertion = lambda do
+          expect(model.new).to validate_presence_of(:attr1, :attr2).allow_nil
+        end
+
+        message = <<-MESSAGE
+Expected Example to validate that :attr2 cannot be empty/falsy, but this
+could not be proved.
+  After setting :attr2 to ‹nil›, the matcher expected the Example to be
+  valid, but it was invalid instead, producing these validation errors:
+
+  * attr2: ["can't be blank"]
+        MESSAGE
+
+        expect(&assertion).to fail_with_message(message)
+      end
+    end
+
     it 'accepts' do
       expect(validating_presence).to matcher
     end