Allow creating merge requests across forks of a project

4 files changed, 34 insertions, 4 deletions

diff --git a/app/finders/merge_request_target_project_finder.rb b/app/finders/merge_request_target_project_finder.rb
new file mode 100644
index 00000000000..508b53a52c1
--- /dev/null
+++ b/app/finders/merge_request_target_project_finder.rb
@@ -0,0 +1,18 @@
+class MergeRequestTargetProjectFinder
+  attr_reader :current_user, :source_project
+
+  def initialize(current_user: nil, source_project:, params: {})
+    @current_user = current_user
+    @source_project = source_project
+  end
+
+  def execute
+    if @source_project.fork_network
+      @source_project.fork_network.projects
+        .public_or_visible_to_user(current_user)
+        .with_feature_available_for_user(:merge_requests, current_user)
+    else
+      Project.where(id: source_project)
+    end
+  end
+end
diff --git a/app/helpers/merge_requests_helper.rb b/app/helpers/merge_requests_helper.rb
index c31023f2d9a..5b2c58d193d 100644
--- a/app/helpers/merge_requests_helper.rb
+++ b/app/helpers/merge_requests_helper.rb
@@ -73,7 +73,8 @@ module MergeRequestsHelper
   end
 
   def target_projects(project)
-    [project, project.default_merge_request_target].uniq
+    MergeRequestTargetProjectFinder.new(current_user: current_user, source_project: project)
+      .execute
   end
 
   def merge_request_button_visibility(merge_request, closed)
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 086226618e6..9b312f7db6c 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -403,7 +403,7 @@ class MergeRequest < ActiveRecord::Base
     return false unless for_fork?
     return true unless source_project
 
-    !source_project.forked_from?(target_project)
+    !source_project.in_fork_network_of?(target_project)
   end
 
   def reopenable?
diff --git a/app/models/project.rb b/app/models/project.rb
index aff8fd768f5..4a883552a8d 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -1128,8 +1128,19 @@ class Project < ActiveRecord::Base
     end
   end
 
-  def in_fork_network_of?(project)
-    forked? && project.fork_network == fork_network
+  def forked_from?(other_project)
+    forked? && forked_from_project == other_project
+  end
+
+  def in_fork_network_of?(other_project)
+    # TODO: Remove this in a next release when all fork_networks are populated
+    # This makes sure all MergeRequests remain valid while the projects don't
+    # have a fork_network yet.
+    return true if forked_from?(other_project)
+
+    return false if fork_network.nil? || other_project.fork_network.nil?
+
+    fork_network == other_project.fork_network
   end
 
   def origin_merge_requests