Fix paginator for Environment Serializer

This commit fixes that the paginator in the Environment Serializer
is not correctly working.

app/serializers/environment_serializer.rb

@@ -23,6 +23,8 @@ class EnvironmentSerializer < BaseSerializer
           latest: super(item.latest, opts) }
       end
     else
+      resource = @paginator.paginate(resource) if paginated?
+
       super(batch_load(resource), opts)
     end
   end
@@ -52,7 +54,7 @@ class EnvironmentSerializer < BaseSerializer
   def batch_load(resource)
     resource = resource.preload(environment_associations)
 
-    resource.all.tap do |environments|
+    resource.all.to_a.tap do |environments|
       environments.each do |environment|
         # Batch loading the commits of the deployments
         environment.last_deployment&.commit&.try(:lazy_author)

changelogs/unreleased/fix-paginator-for-environment-serializer.yml

+---
+title: Fix paginator of Environment Serializer
+merge_request: 59751
+author:
+type: fixed

spec/support/shared_examples/serializers/environment_serializer_shared_examples.rb

@@ -20,9 +20,27 @@ RSpec.shared_examples 'avoid N+1 on environments serialization' do
     expect { serialize(grouping: false) }.not_to exceed_query_limit(control.count)
   end
 
-  def serialize(grouping:)
+  it 'does not preload for environments that does not exist in the page', :request_store do
+    create_environment_with_associations(project)
+
+    first_page_query = ActiveRecord::QueryRecorder.new do
+      serialize(grouping: false, query: { page: 1, per_page: 1 } )
+    end
+
+    second_page_query = ActiveRecord::QueryRecorder.new do
+      serialize(grouping: false, query: { page: 2, per_page: 1 } )
+    end
+
+    expect(second_page_query.count).to be < first_page_query.count
+  end
+
+  def serialize(grouping:, query: nil)
+    query ||= { page: 1, per_page: 1 }
+    request = double(url: "#{Gitlab.config.gitlab.url}:8080/api/v4/projects?#{query.to_query}", query_parameters: query)
+
     EnvironmentSerializer.new(current_user: user, project: project).yield_self do |serializer|
       serializer.within_folders if grouping
+      serializer.with_pagination(request, spy('response'))
       serializer.represent(Environment.where(project: project))
     end
   end