diff --git a/changelogs/unreleased/32283-trending-projects-unique-constraint2.yml b/changelogs/unreleased/32283-trending-projects-unique-constraint2.yml
new file mode 100644
index 00000000000..4fd6b6fddc4
--- /dev/null
+++ b/changelogs/unreleased/32283-trending-projects-unique-constraint2.yml
@@ -0,0 +1,5 @@
+---
+title: Add unique constraint to trending_projects#project_id.
+merge_request: 16846
+author:
+type: other
diff --git a/db/migrate/20180201102129_add_unique_constraint_to_trending_projects_project_id.rb b/db/migrate/20180201102129_add_unique_constraint_to_trending_projects_project_id.rb
new file mode 100644
index 00000000000..02e53b8fa8a
--- /dev/null
+++ b/db/migrate/20180201102129_add_unique_constraint_to_trending_projects_project_id.rb
@@ -0,0 +1,19 @@
+class AddUniqueConstraintToTrendingProjectsProjectId < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :trending_projects, :project_id, unique: true, name: 'index_trending_projects_on_project_id_unique'
+    remove_concurrent_index_by_name :trending_projects, 'index_trending_projects_on_project_id'
+    rename_index :trending_projects, 'index_trending_projects_on_project_id_unique', 'index_trending_projects_on_project_id'
+  end
+
+  def down
+    rename_index :trending_projects, 'index_trending_projects_on_project_id', 'index_trending_projects_on_project_id_old'
+    add_concurrent_index :trending_projects, :project_id
+    remove_concurrent_index_by_name :trending_projects, 'index_trending_projects_on_project_id_old'
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index f72989ff391..c701a5f1e17 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -1727,7 +1727,7 @@ ActiveRecord::Schema.define(version: 20180202111106) do
     t.integer "project_id", null: false
   end
 
-  add_index "trending_projects", ["project_id"], name: "index_trending_projects_on_project_id", using: :btree
+  add_index "trending_projects", ["project_id"], name: "index_trending_projects_on_project_id", unique: true, using: :btree
 
   create_table "u2f_registrations", force: :cascade do |t|
     t.text "certificate"