Added RuboCop cops for checking DB migrations

.rubocop.yml

-require: rubocop-rspec
+require:
+  - rubocop-rspec
+  - ./rubocop/rubocop
 
 AllCops:
   TargetRubyVersion: 2.1

rubocop/cop/migration/add_index.rb

+module RuboCop
+  module Cop
+    module Migration
+      # Cop that checks if indexes are added in a concurrent manner.
+      class AddIndex < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        MSG = 'add_index requires downtime, use add_concurrent_index instead'
+
+        def on_def(node)
+          return unless in_migration?(node)
+
+          new_tables = []
+
+          node.each_descendant(:send) do |send_node|
+            first_arg = first_argument(send_node)
+
+            # The first argument of "create_table" / "add_index" is the table
+            # name.
+            new_tables << first_arg if create_table?(send_node)
+
+            next if method_name(send_node) != :add_index
+
+            # Using "add_index" is fine for newly created tables as there's no
+            # data in these tables yet.
+            next if new_tables.include?(first_arg)
+
+            add_offense(send_node, :selector)
+          end
+        end
+
+        def create_table?(node)
+          method_name(node) == :create_table
+        end
+
+        def method_name(node)
+          node.children[1]
+        end
+
+        def first_argument(node)
+          node.children[2] ? node.children[0] : nil
+        end
+      end
+    end
+  end
+end

rubocop/cop/migration/column_with_default.rb

+module RuboCop
+  module Cop
+    module Migration
+      # Cop that checks if columns are added in a way that doesn't require
+      # downtime.
+      class ColumnWithDefault < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        WHITELISTED_TABLES = [:application_settings]
+
+        MSG = 'add_column with a default value requires downtime, ' \
+          'use add_column_with_default instead'
+
+        def on_send(node)
+          return unless in_migration?(node)
+
+          name = node.children[1]
+
+          return unless name == :add_column
+
+          # Ignore whitelisted tables.
+          return if table_whitelisted?(node.children[2])
+
+          opts = node.children.last
+
+          return unless opts && opts.type == :hash
+
+          opts.each_node(:pair) do |pair|
+            if hash_key_type(pair) == :sym && hash_key_name(pair) == :default
+              add_offense(node, :selector)
+            end
+          end
+        end
+
+        def table_whitelisted?(symbol)
+          symbol && symbol.type == :sym &&
+            WHITELISTED_TABLES.include?(symbol.children[0])
+        end
+
+        def hash_key_type(pair)
+          pair.children[0].type
+        end
+
+        def hash_key_name(pair)
+          pair.children[0].children[0]
+        end
+      end
+    end
+  end
+end

rubocop/migration_helpers.rb

+module RuboCop
+  # Module containing helper methods for writing migration cops.
+  module MigrationHelpers
+    # Returns true if the given node originated from the db/migrate directory.
+    def in_migration?(node)
+      File.dirname(node.location.expression.source_buffer.name).
+        end_with?('db/migrate')
+    end
+  end
+end

rubocop/rubocop.rb

+require_relative 'migration_helpers'
+require_relative 'cop/migration/add_index'
+require_relative 'cop/migration/column_with_default'