Add table for the design<->version relation

This replaces the single design a version can belong_to a setup
where a version can belong to multiple designs.

The constraints should enforce that we only create a single version,
and only one row in the table linking them to a design.

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20190328210840) do
+ActiveRecord::Schema.define(version: 20190403161806) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1042,14 +1042,19 @@ ActiveRecord::Schema.define(version: 20190328210840) do
     t.integer "issue_id", null: false
     t.string "filename", null: false
     t.index ["issue_id", "filename"], name: "index_design_management_designs_on_issue_id_and_filename", unique: true, using: :btree
-    t.index ["issue_id"], name: "index_design_management_designs_on_issue_id", unique: true, using: :btree
     t.index ["project_id"], name: "index_design_management_designs_on_project_id", using: :btree
   end
 
+  create_table "design_management_designs_versions", id: false, force: :cascade do |t|
+    t.bigint "design_id", null: false
+    t.bigint "version_id", null: false
+    t.index ["design_id", "version_id"], name: "design_management_designs_versions_uniqueness", unique: true, using: :btree
+    t.index ["design_id"], name: "index_design_management_designs_versions_on_design_id", using: :btree
+    t.index ["version_id"], name: "index_design_management_designs_versions_on_version_id", using: :btree
+  end
+
   create_table "design_management_versions", id: :bigserial, force: :cascade do |t|
-    t.bigint "design_management_design_id", null: false
     t.binary "sha", null: false
-    t.index ["design_management_design_id"], name: "index_design_management_versions_on_design_management_design_id", using: :btree
     t.index ["sha"], name: "index_design_management_versions_on_sha", unique: true, using: :btree
   end
 
@@ -3475,7 +3480,8 @@ ActiveRecord::Schema.define(version: 20190328210840) do
   add_foreign_key "deployments", "projects", name: "fk_b9a3851b82", on_delete: :cascade
   add_foreign_key "design_management_designs", "issues", on_delete: :cascade
   add_foreign_key "design_management_designs", "projects", on_delete: :cascade
-  add_foreign_key "design_management_versions", "design_management_designs", on_delete: :cascade
+  add_foreign_key "design_management_designs_versions", "design_management_designs", column: "design_id", on_delete: :cascade
+  add_foreign_key "design_management_designs_versions", "design_management_versions", column: "version_id", on_delete: :cascade
   add_foreign_key "draft_notes", "merge_requests", on_delete: :cascade
   add_foreign_key "draft_notes", "users", column: "author_id", on_delete: :cascade
   add_foreign_key "elasticsearch_indexed_namespaces", "namespaces", on_delete: :cascade

ee/db/migrate/20190402112450_add_design_management_designs_versions.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddDesignManagementDesignsVersions < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  # When a migration requires downtime you **must** uncomment the following
+  # constant and define a short and easy to understand explanation as to why the
+  # migration requires downtime.
+  # DOWNTIME_REASON = ''
+
+  # When using the methods "add_concurrent_index", "remove_concurrent_index" or
+  # "add_column_with_default" you must disable the use of transactions
+  # as these methods can not run in an existing transaction.
+  # When using "add_concurrent_index" or "remove_concurrent_index" methods make sure
+  # that either of them is the _only_ method called in the migration,
+  # any other changes should go in a separate migration.
+  # This ensures that upon failure _only_ the index creation or removing fails
+  # and can be retried or reverted easily.
+  #
+  # To disable transactions uncomment the following line and remove these
+  # comments:
+  # disable_ddl_transaction!
+
+  def change
+    create_table(:design_management_designs_versions, id: false) do |t|
+      t.references :design,
+                   null: false,
+                   type: :bigint,
+                   foreign_key: {
+                     on_delete: :cascade,
+                     to_table: :design_management_designs
+                   }
+      t.references :version,
+                   null: false,
+                   type: :bigint,
+                   foreign_key: {
+                     on_delete: :cascade,
+                     to_table: :design_management_versions
+                   }
+    end
+
+    add_index :design_management_designs_versions,
+              [:design_id, :version_id],
+              unique: true, name: "design_management_designs_versions_uniqueness"
+  end
+end

ee/db/migrate/20190403161806_update_designs_index.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class UpdateDesignsIndex < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index :design_management_designs, :issue_id, unique: true
+  end
+
+  def down
+    add_concurrent_index :design_management_designs, :issue_id, unique: true
+  end
+end

ee/db/post_migrate/20190402111436_remove_design_id_from_design_versions.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class RemoveDesignIdFromDesignVersions < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    remove_foreign_key :design_management_versions, :design_management_designs
+    remove_column(:design_management_versions, :design_management_design_id)
+  end
+
+  def down
+    add_column(:design_management_versions, :design_management_design_id, :bigint)
+    add_concurrent_foreign_key :design_management_versions, :design_management_designs, column: :design_management_design_id
+  end
+end