Remove duplicated records and add unique constraint

db/migrate/20170622135451_remove_duplicated_variable.rb

+class RemoveDuplicatedVariable < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    if Gitlab::Database.postgresql?
+      execute <<~SQL
+        DELETE FROM ci_variables var USING (#{duplicated_ids}) dup
+          #{join_conditions}
+      SQL
+    else
+      execute <<~SQL
+        DELETE var FROM ci_variables var INNER JOIN (#{duplicated_ids}) dup
+          #{join_conditions}
+      SQL
+    end
+  end
+
+  def down
+    # noop
+  end
+
+  def duplicated_ids
+    <<~SQL
+      SELECT MAX(id) AS id, #{key}, project_id
+        FROM ci_variables GROUP BY #{key}, project_id
+    SQL
+  end
+
+  def join_conditions
+    <<~SQL
+      WHERE var.key = dup.key
+        AND var.project_id = dup.project_id
+        AND var.id <> dup.id
+    SQL
+  end
+
+  def key
+    # key needs to be quoted in MySQL
+    quote_column_name('key')
+  end
+end

db/migrate/20170622135728_add_unique_constraint_to_ci_variables.rb

+class AddUniqueConstraintToCiVariables < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    unless index_exists?(:ci_variables, columns)
+      add_concurrent_index(:ci_variables, columns, unique: true)
+    end
+  end
+
+  def down
+    if index_exists?(:ci_variables, columns)
+      remove_concurrent_index(:ci_variables, columns)
+    end
+  end
+
+  def columns
+    @columns ||= [:project_id, :key, :environment_scope]
+  end
+end

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: 20170621102400) do
+ActiveRecord::Schema.define(version: 20170622135728) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -377,7 +377,7 @@ ActiveRecord::Schema.define(version: 20170621102400) do
     t.string "environment_scope", default: "*", null: false
   end
 
-  add_index "ci_variables", ["project_id"], name: "index_ci_variables_on_project_id", using: :btree
+  add_index "ci_variables", ["project_id", "key", "environment_scope"], name: "index_ci_variables_on_project_id_and_key_and_environment_scope", unique: true, using: :btree
 
   create_table "container_repositories", force: :cascade do |t|
     t.integer "project_id", null: false

spec/migrations/remove_duplicated_variable_spec.rb

+require 'spec_helper'
+require Rails.root.join('db', 'migrate', '20170622135451_remove_duplicated_variable.rb')
+
+describe RemoveDuplicatedVariable, :migration do
+  let(:variables) { table(:ci_variables) }
+  let(:projects) { table(:projects) }
+
+  before do
+    projects.create!(id: 1)
+    variables.create!(id: 1, key: 'key1', project_id: 1)
+    variables.create!(id: 2, key: 'key2', project_id: 1)
+    variables.create!(id: 3, key: 'keyX', project_id: 1)
+    variables.create!(id: 4, key: 'keyX', project_id: 1)
+    variables.create!(id: 5, key: 'keyY', project_id: 1)
+    variables.create!(id: 6, key: 'keyX', project_id: 1)
+    variables.create!(id: 7, key: 'key7', project_id: 1)
+    variables.create!(id: 8, key: 'keyY', project_id: 1)
+  end
+
+  it 'correctly remove duplicated records with smaller id' do
+    migrate!
+
+    expect(variables.pluck(:id)).to contain_exactly(1, 2, 6, 7, 8)
+  end
+end