A project can have only one project_repository

db/migrate/20181122160027_create_project_repositories.rb

@@ -12,7 +12,7 @@ class CreateProjectRepositories < ActiveRecord::Migration[5.0]
     create_table :project_repositories, id: :bigserial do |t|
       t.references :shard, null: false, index: true, foreign_key: { on_delete: :restrict }
       t.string :disk_path, null: false, index: { unique: true }
-      t.references :project, null: false, index: true, foreign_key: { on_delete: :cascade }
+      t.references :project, null: false, index: { unique: true }, foreign_key: { on_delete: :cascade }
     end
   end
 end

db/schema.rb

@@ -2154,7 +2154,7 @@ ActiveRecord::Schema.define(version: 20181126153547) do
     t.string "disk_path", null: false
     t.integer "project_id", null: false
     t.index ["disk_path"], name: "index_project_repositories_on_disk_path", unique: true, using: :btree
-    t.index ["project_id"], name: "index_project_repositories_on_project_id", using: :btree
+    t.index ["project_id"], name: "index_project_repositories_on_project_id", unique: true, using: :btree
     t.index ["shard_id"], name: "index_project_repositories_on_shard_id", using: :btree
   end