diff --git a/db/migrate/20170713104829_add_foreign_key_to_merge_requests.rb b/db/migrate/20170713104829_add_foreign_key_to_merge_requests.rb
new file mode 100644
index 0000000000000000000000000000000000000000..78ff75279679a2734a55fb6c9ff1abdaa23c5256
--- /dev/null
+++ b/db/migrate/20170713104829_add_foreign_key_to_merge_requests.rb
@@ -0,0 +1,28 @@
+class AddForeignKeyToMergeRequests < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    unless foreign_key_exists?(:merge_requests, :head_pipeline_id)
+      add_concurrent_foreign_key(:merge_requests, :ci_pipelines,
+                                 column: :head_pipeline_id, on_delete: :nullify)
+    end
+  end
+
+  def down
+    if foreign_key_exists?(:merge_requests, :head_pipeline_id)
+      remove_foreign_key(:merge_requests, column: :head_pipeline_id)
+    end
+  end
+
+  private
+
+  def foreign_key_exists?(table, column)
+    foreign_keys(table).any? do |key|
+      key.options[:column] == column.to_s
+    end
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 5264fc995579235209bd6c28eb7f020e00cbeef7..9c8c64fe2d05fdc59d4bc50040634b286fe2ece7 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170707184244) do
+ActiveRecord::Schema.define(version: 20170713104829) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1615,6 +1615,7 @@ ActiveRecord::Schema.define(version: 20170707184244) do
   add_foreign_key "merge_request_diffs", "merge_requests", name: "fk_8483f3258f", on_delete: :cascade
   add_foreign_key "merge_request_metrics", "ci_pipelines", column: "pipeline_id", on_delete: :cascade
   add_foreign_key "merge_request_metrics", "merge_requests", on_delete: :cascade
+  add_foreign_key "merge_requests", "ci_pipelines", column: "head_pipeline_id", name: "fk_fd82eae0b9", on_delete: :nullify
   add_foreign_key "merge_requests", "projects", column: "target_project_id", name: "fk_a6963e8447", on_delete: :cascade
   add_foreign_key "merge_requests_closing_issues", "issues", on_delete: :cascade
   add_foreign_key "merge_requests_closing_issues", "merge_requests", on_delete: :cascade
diff --git a/lib/gitlab/database/migration_helpers.rb b/lib/gitlab/database/migration_helpers.rb
index 0643c56db9bc7d53f4cf0762a1b7a10c84ae153b..69ca9aa596bac64da820ae4868a3f5a8047320fb 100644
--- a/lib/gitlab/database/migration_helpers.rb
+++ b/lib/gitlab/database/migration_helpers.rb
@@ -140,6 +140,8 @@ module Gitlab
           return add_foreign_key(source, target,
                                  column: column,
                                  on_delete: on_delete)
+        else
+          on_delete = 'SET NULL' if on_delete == :nullify
         end
 
         disable_statement_timeout
@@ -155,7 +157,7 @@ module Gitlab
         ADD CONSTRAINT #{key_name}
         FOREIGN KEY (#{column})
         REFERENCES #{target} (id)
-        #{on_delete ? "ON DELETE #{on_delete}" : ''}
+        #{on_delete ? "ON DELETE #{on_delete.upcase}" : ''}
         NOT VALID;
         EOF
 
diff --git a/spec/lib/gitlab/database/migration_helpers_spec.rb b/spec/lib/gitlab/database/migration_helpers_spec.rb
index 4259be3f522240e97c630c9ccac868cbcbb52e3f..a2acd15c8fb7940d1c3c60ccce3d8f0ef6700294 100644
--- a/spec/lib/gitlab/database/migration_helpers_spec.rb
+++ b/spec/lib/gitlab/database/migration_helpers_spec.rb
@@ -174,13 +174,23 @@ describe Gitlab::Database::MigrationHelpers, lib: true do
           allow(Gitlab::Database).to receive(:mysql?).and_return(false)
         end
 
-        it 'creates a concurrent foreign key' do
+        it 'creates a concurrent foreign key and validates it' do
           expect(model).to receive(:disable_statement_timeout)
           expect(model).to receive(:execute).ordered.with(/NOT VALID/)
           expect(model).to receive(:execute).ordered.with(/VALIDATE CONSTRAINT/)
 
           model.add_concurrent_foreign_key(:projects, :users, column: :user_id)
         end
+
+        it 'appends a valid ON DELETE statement' do
+          expect(model).to receive(:disable_statement_timeout)
+          expect(model).to receive(:execute).with(/ON DELETE SET NULL/)
+          expect(model).to receive(:execute).ordered.with(/VALIDATE CONSTRAINT/)
+
+          model.add_concurrent_foreign_key(:projects, :users,
+                                           column: :user_id,
+                                           on_delete: :nullify)
+        end
       end
     end
   end