Add field mergeRequests for project in GraphQL

And fix the tests so that it won't run into circular paths.

app/graphql/mutations/merge_requests/base.rb

@@ -25,7 +25,8 @@ module Mutations
       def find_object(project_path:, iid:)
         project = resolve_project(full_path: project_path)
-        resolver = Resolvers::MergeRequestResolver.new(object: project, context: context)
+        resolver = Resolvers::MergeRequestsResolver
+          .single.new(object: project, context: context)
         resolver.resolve(iid: iid)
       end

app/graphql/resolvers/base_resolver.rb

@@ -2,5 +2,12 @@
 module Resolvers
   class BaseResolver < GraphQL::Schema::Resolver
+    def self.single
+      @single ||= Class.new(self) do
+        def resolve(**args)
+          super.first
+        end
+      end
+    end
   end
 end

app/graphql/resolvers/merge_request_resolver.rb → app/graphql/resolvers/merge_requests_resolver.rb

 # frozen_string_literal: true
 module Resolvers
-  class MergeRequestResolver < BaseResolver
+  class MergeRequestsResolver < BaseResolver
     argument :iid, GraphQL::ID_TYPE,
-             required: true,
+             required: false,
              description: 'The IID of the merge request, e.g., "1"'
+    argument :iids, [GraphQL::ID_TYPE],
+              required: false,
+              description: 'The list of IIDs of issues, e.g., [1, 2]'
+
     type Types::MergeRequestType, null: true
     alias_method :project, :object
-    # rubocop: disable CodeReuse/ActiveRecord
-    def resolve(iid:)
+    def resolve(**args)
       return unless project.present?
+      args[:iids] ||= [args[:iid]]
+
+      args[:iids].map(&method(:batch_load))
+        .select(&:itself) # .compact doesn't work on BatchLoader
+    end
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def batch_load(iid)
       BatchLoader.for(iid.to_s).batch(key: project) do |iids, loader, args|
         args[:key].merge_requests.where(iid: iids).each do |mr|
           loader.call(mr.iid.to_s, mr)

app/graphql/types/project_type.rb

@@ -66,10 +66,17 @@ module Types
     field :only_allow_merge_if_all_discussions_are_resolved, GraphQL::BOOLEAN_TYPE, null: true
     field :printing_merge_request_link_enabled, GraphQL::BOOLEAN_TYPE, null: true
+    field :merge_requests,
+          Types::MergeRequestType.connection_type,
+          null: true,
+          resolver: Resolvers::MergeRequestsResolver do
+      # authorize :read_merge_request
+    end
+
     field :merge_request,
           Types::MergeRequestType,
           null: true,
-          resolver: Resolvers::MergeRequestResolver do
+          resolver: Resolvers::MergeRequestsResolver.single do
       authorize :read_merge_request
     end

changelogs/unreleased/56485-implement-graphql-mergerequestsresolver.yml

+---
+title: Add field mergeRequests for project in GraphQL
+merge_request: 24805
+author:
+type: added

spec/graphql/resolvers/merge_request_resolver_spec.rb → spec/graphql/resolvers/merge_requests_resolver_spec.rb

 require 'spec_helper'
-describe Resolvers::MergeRequestResolver do
+describe Resolvers::MergeRequestsResolver do
   include GraphqlHelpers
   set(:project) { create(:project, :repository) }
@@ -18,7 +18,7 @@ describe Resolvers::MergeRequestResolver do
   describe '#resolve' do
     it 'batch-resolves merge requests by target project full path and IID' do
       result = batch(max_queries: 2) do
-        [resolve_mr(project, iid_1), resolve_mr(project, iid_2)]
+        resolve_mr(project, iid_1) + resolve_mr(project, iid_2)
       end
       expect(result).to contain_exactly(merge_request_1, merge_request_2)
@@ -26,7 +26,9 @@ describe Resolvers::MergeRequestResolver do
     it 'can batch-resolve merge requests from different projects' do
       result = batch(max_queries: 3) do
-        [resolve_mr(project, iid_1), resolve_mr(project, iid_2), resolve_mr(other_project, other_iid)]
+        resolve_mr(project, iid_1) +
+          resolve_mr(project, iid_2) +
+          resolve_mr(other_project, other_iid)
       end
       expect(result).to contain_exactly(merge_request_1, merge_request_2, other_merge_request)
@@ -35,7 +37,7 @@ describe Resolvers::MergeRequestResolver do
     it 'resolves an unknown iid to nil' do
       result = batch { resolve_mr(project, -1) }
-      expect(result).to be_nil
+      expect(result).to be_empty
     end
   end

spec/support/helpers/graphql_helpers.rb

@@ -84,7 +84,7 @@ module GraphqlHelpers
     QUERY
   end
-  def all_graphql_fields_for(class_name)
+  def all_graphql_fields_for(class_name, parent_types = Set.new)
     type = GitlabSchema.types[class_name.to_s]
     return "" unless type
@@ -92,8 +92,17 @@ module GraphqlHelpers
       # We can't guess arguments, so skip fields that require them
       next if required_arguments?(field)
+      singular_field_type = field_type(field)
+
+      # If field type is the same as parent type, then we're hitting into
+      # mutual dependency. Break it from infinite recursion
+      next if parent_types.include?(singular_field_type)
+
       if nested_fields?(field)
-        "#{name} { #{all_graphql_fields_for(field_type(field))} }"
+        fields =
+          all_graphql_fields_for(singular_field_type, parent_types | [type])
+
+        "#{name} { #{fields} }"
       else
         name
       end