3 files changed, 134 insertions, 0 deletions
diff --git a/app/graphql/resolvers/concerns/looks_ahead.rb b/app/graphql/resolvers/concerns/looks_ahead.rb
new file mode 100644
index 00000000000..becc6debd33
--- /dev/null
+++ b/app/graphql/resolvers/concerns/looks_ahead.rb
@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+module LooksAhead
+  extend ActiveSupport::Concern
+
+  FEATURE_FLAG = :graphql_lookahead_support
+
+  included do
+    attr_accessor :lookahead
+  end
+
+  def resolve(**args)
+    self.lookahead = args.delete(:lookahead)
+
+    resolve_with_lookahead(**args)
+  end
+
+  def apply_lookahead(query)
+    return query unless Feature.enabled?(FEATURE_FLAG)
+
+    selection = node_selection
+
+    includes = preloads.each.flat_map do |name, requirements|
+      selection&.selects?(name) ? requirements : []
+    end
+    preloads = (unconditional_includes + includes).uniq
+
+    return query if preloads.empty?
+
+    query.preload(*preloads) # rubocop: disable CodeReuse/ActiveRecord
+  end
+
+  private
+
+  def unconditional_includes
+    []
+  end
+
+  def preloads
+    {}
+  end
+
+  def node_selection
+    return unless lookahead
+
+    if lookahead.selects?(:nodes)
+      lookahead.selection(:nodes)
+    elsif lookahead.selects?(:edges)
+      lookahead.selection(:edges).selection(:nodes)
+    end
+  end
+end
diff --git a/app/graphql/resolvers/concerns/resolves_merge_requests.rb b/app/graphql/resolvers/concerns/resolves_merge_requests.rb
new file mode 100644
index 00000000000..a2140728a27
--- /dev/null
+++ b/app/graphql/resolvers/concerns/resolves_merge_requests.rb
@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+# Mixin for resolving merge requests. All arguments must be in forms
+# that `MergeRequestsFinder` can handle, so you may need to use aliasing.
+module ResolvesMergeRequests
+  extend ActiveSupport::Concern
+  include LooksAhead
+
+  included do
+    type Types::MergeRequestType, null: true
+  end
+
+  def resolve_with_lookahead(**args)
+    args[:iids] = Array.wrap(args[:iids]) if args[:iids]
+    args.compact!
+
+    if project && args.keys == [:iids]
+      batch_load_merge_requests(args[:iids])
+    else
+      args[:project_id] ||= project
+
+      apply_lookahead(MergeRequestsFinder.new(current_user, args).execute)
+    end.then(&(single? ? :first : :itself))
+  end
+
+  def ready?(**args)
+    return early_return if no_results_possible?(args)
+
+    super
+  end
+
+  def early_return
+    [false, single? ? nil : MergeRequest.none]
+  end
+
+  private
+
+  def batch_load_merge_requests(iids)
+    iids.map { |iid| batch_load(iid) }.select(&:itself) # .compact doesn't work on BatchLoader
+  end
+
+  # rubocop: disable CodeReuse/ActiveRecord
+  def batch_load(iid)
+    BatchLoader::GraphQL.for(iid.to_s).batch(key: project) do |iids, loader, args|
+      query = args[:key].merge_requests.where(iid: iids)
+
+      apply_lookahead(query).each do |mr|
+        loader.call(mr.iid.to_s, mr)
+      end
+    end
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
+
+  def unconditional_includes
+    [:target_project]
+  end
+
+  def preloads
+    {
+      assignees: [:assignees],
+      labels: [:labels],
+      author: [:author],
+      milestone: [:milestone],
+      head_pipeline: [:merge_request_diff, { head_pipeline: [:merge_request] }]
+    }
+  end
+end
diff --git a/app/graphql/resolvers/concerns/resolves_project.rb b/app/graphql/resolvers/concerns/resolves_project.rb
new file mode 100644
index 00000000000..3c5ce3dab01
--- /dev/null
+++ b/app/graphql/resolvers/concerns/resolves_project.rb
@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module ResolvesProject
+  def resolve_project(full_path: nil, project_id: nil)
+    unless full_path.present? ^ project_id.present?
+      raise ::Gitlab::Graphql::Errors::ArgumentError, 'Incompatible arguments: projectId, projectPath.'
+    end
+
+    if full_path.present?
+      ::Gitlab::Graphql::Loaders::FullPathModelLoader.new(Project, full_path).find
+    else
+      ::GitlabSchema.object_from_id(project_id, expected_type: Project)
+    end
+  end
+end