Merge branch 'fix-remove-merge-request-diff-unique-index' into 'master'

This MR fixes the migration that removes the unique index/constraint from `merge_request_diffs.merge_request_id`

See merge request !6346

db/migrate/20160725104020_merge_request_diff_remove_uniq.rb

@@ -8,14 +8,28 @@ class MergeRequestDiffRemoveUniq < ActiveRecord::Migration
   DOWNTIME = false
 
   def up
+    constraint_name = 'merge_request_diffs_merge_request_id_key'
+
+    transaction do
       if index_exists?(:merge_request_diffs, :merge_request_id)
-      remove_index :merge_request_diffs, :merge_request_id
+        remove_index(:merge_request_diffs, :merge_request_id)
+      end
+
+      # In some bizarre cases PostgreSQL might have a separate unique constraint
+      # that we'll need to drop.
+      if constraint_exists?(constraint_name) && Gitlab::Database.postgresql?
+        execute("ALTER TABLE merge_request_diffs DROP CONSTRAINT IF EXISTS #{constraint_name};")
+      end
     end
   end
 
   def down
     unless index_exists?(:merge_request_diffs, :merge_request_id)
-      add_concurrent_index :merge_request_diffs, :merge_request_id, unique: true
+      add_concurrent_index(:merge_request_diffs, :merge_request_id, unique: true)
+    end
   end
+
+  def constraint_exists?(name)
+    indexes(:merge_request_diffs).map(&:name).include?(name)
   end
 end