Add resolvers for the user <-> merge-request edges

These resolvers re-use the functionality of the merge_requests_resolver
to find merge requests either authored by or assigned to a user. The
author is the implicit argument of the edge, and they support all the
filtering arguments of the merge_requests_resolver, as well as being
able to restrict the project, either by ID or full-path.

app/graphql/resolvers/assigned_merge_requests_resolver.rb

+# frozen_string_literal: true
+
+module Resolvers
+  class AssignedMergeRequestsResolver < UserMergeRequestsResolver
+    def user_role
+      :assignee
+    end
+  end
+end

app/graphql/resolvers/authored_merge_requests_resolver.rb

+# frozen_string_literal: true
+
+module Resolvers
+  class AuthoredMergeRequestsResolver < UserMergeRequestsResolver
+    def user_role
+      :author
+    end
+  end
+end

app/graphql/resolvers/concerns/resolves_merge_requests.rb

@@ -13,10 +13,10 @@ module ResolvesMergeRequests
     args[:iids] = Array.wrap(args[:iids]) if args[:iids]
     args.compact!
 
-    if args.keys == [:iids]
+    if project && args.keys == [:iids]
       batch_load_merge_requests(args[:iids])
     else
-      args[:project_id] = project.id
+      args[:project_id] ||= project
 
       MergeRequestsFinder.new(current_user, args).execute
     end.then(&(single? ? :first : :itself))

app/graphql/resolvers/merge_requests_resolver.rb

@@ -34,7 +34,11 @@ module Resolvers
     end
 
     def no_results_possible?(args)
-      project.nil? || args.values.any? { |v| v.is_a?(Array) && v.empty? }
+      project.nil? || some_argument_is_empty?(args)
+    end
+
+    def some_argument_is_empty?(args)
+      args.values.any? { |v| v.is_a?(Array) && v.empty? }
     end
   end
 end

app/graphql/resolvers/user_merge_requests_resolver.rb

+# frozen_string_literal: true
+
+module Resolvers
+  class UserMergeRequestsResolver < MergeRequestsResolver
+    include ResolvesProject
+
+    argument :project_path, GraphQL::STRING_TYPE,
+          required: false,
+          description: 'The full-path of the project the authored merge requests should be in. Incompatible with projectId.'
+
+    argument :project_id, GraphQL::ID_TYPE,
+          required: false,
+          description: 'The global ID of the project the authored merge requests should be in. Incompatible with projectPath.'
+
+    attr_reader :project
+    alias_method :user, :synchronized_object
+
+    def ready?(project_id: nil, project_path: nil, **args)
+      return early_return unless can_read_profile?
+
+      if project_id || project_path
+        load_project(project_path, project_id)
+        return early_return unless can_read_project?
+      elsif args[:iids].present?
+        raise ::Gitlab::Graphql::Errors::ArgumentError,
+          'iids requires projectPath or projectId'
+      end
+
+      super(**args)
+    end
+
+    def resolve(**args)
+      prepare_args(args)
+      key = :"#{user_role}_id"
+      super(key => user.id, **args)
+    end
+
+    def user_role
+      raise NotImplementedError
+    end
+
+    private
+
+    def can_read_profile?
+      Ability.allowed?(current_user, :read_user_profile, user)
+    end
+
+    def can_read_project?
+      Ability.allowed?(current_user, :read_merge_request, project)
+    end
+
+    def load_project(project_path, project_id)
+      @project = resolve_project(full_path: project_path, project_id: project_id)
+      @project = @project.sync if @project.respond_to?(:sync)
+    end
+
+    def no_results_possible?(args)
+      some_argument_is_empty?(args)
+    end
+
+    # These arguments are handled in load_project, and should not be passed to
+    # the finder directly.
+    def prepare_args(args)
+      args.delete(:project_id)
+      args.delete(:project_path)
+    end
+  end
+end

app/graphql/resolvers/user_resolver.rb

@@ -4,6 +4,8 @@ module Resolvers
   class UserResolver < BaseResolver
     description 'Retrieve a single user'
 
+    type Types::UserType, null: true
+
     argument :id, GraphQL::ID_TYPE,
              required: false,
              description: 'ID of the User'
@@ -12,13 +14,6 @@ module Resolvers
              required: false,
              description: 'Username of the User'
 
-    def resolve(id: nil, username: nil)
-      id_or_username = GitlabSchema.parse_gid(id, expected_type: ::User).model_id if id
-      id_or_username ||= username
-
-      ::UserFinder.new(id_or_username).find_by_id_or_username
-    end
-
     def ready?(id: nil, username: nil)
       unless id.present? ^ username.present?
         raise Gitlab::Graphql::Errors::ArgumentError, 'Provide either a single username or id'
@@ -26,5 +21,23 @@ module Resolvers
 
       super
     end
+
+    def resolve(id: nil, username: nil)
+      if id
+        GitlabSchema.object_from_id(id, expected_type: User)
+      else
+        batch_load(username)
+      end
+    end
+
+    private
+
+    def batch_load(username)
+      BatchLoader::GraphQL.for(username).batch do |usernames, loader|
+        User.by_username(usernames).each do |user|
+          loader.call(user.username, user)
+        end
+      end
+    end
   end
 end