Add change_column_safely migration helper

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 - Added `add_foreign_key_safely` method
 - Added `add_null_constraint_safely` and `remove_null_constraint_safely` methods
+- Added `add_column_safely` method
+- Added `backfill_column_safely` method
+- Added `change_column_safely` method
 - Added `statement_timeout` and `lock_timeout` functionality
 
 ## 0.5.1 (2019-12-17)

lib/strong_migrations.rb

@@ -2,6 +2,7 @@
 require "active_support"
 
 # modules
+require "strong_migrations/util"
 require "strong_migrations/checker"
 require "strong_migrations/database_tasks"
 require "strong_migrations/migration"
@@ -25,28 +26,15 @@ class << self
   self.error_messages = {
     add_column_default:
 "Adding a column with a non-null default causes the entire table to be rewritten.
-Instead, add the column without a default value, then change the default.
+Instead, add the column without a default value, backfill and then change the default.
 
 class %{migration_name} < ActiveRecord::Migration%{migration_suffix}
-  def up
-    %{add_command}
-    %{change_command}
-  end
-
-  def down
-    %{remove_command}
-  end
-end
-
-Then backfill the existing rows in the Rails console or a separate migration with disable_ddl_transaction!.
-
-class Backfill%{migration_name} < ActiveRecord::Migration%{migration_suffix}
   disable_ddl_transaction!
 
   def change
-    %{code}
+    %{command}
   end
-end%{append}",
+end",
 
     add_column_json:
 "There's no equality operator for the json column type, which can
@@ -61,7 +49,21 @@ def change
 3. Backfill data from the old column to the new column
 4. Move reads from the old column to the new column
 5. Stop writing to the old column
-6. Drop the old column",
+6. Drop the old column
+
+To achieve this, you can use:
+
+class %{migration_name} < ActiveRecord::Migration%{migration_suffix}
+  disable_ddl_transaction!
+
+  def up
+    %{up_command}
+  end
+
+  def down
+    %{down_command}
+  end
+end",
 
     remove_column: "ActiveRecord caches attributes which causes problems
 when removing columns. Be sure to ignore the column%{column_suffix}:

lib/strong_migrations/checker.rb

@@ -1,5 +1,7 @@
 module StrongMigrations
   class Checker
+    include Util
+
     attr_accessor :direction
 
     def initialize(migration)
@@ -81,35 +83,39 @@ def perform(method, *args)
           default = options[:default]
 
           if !default.nil? && !(postgresql? && postgresql_version >= 110000)
-
-            if options[:null] == false
-              options = options.except(:null)
-              append = "
-
-Then add the NOT NULL constraint."
-            end
-
             raise_error :add_column_default,
-              add_command: command_str("add_column", [table, column, type, options.except(:default)]),
-              change_command: command_str("change_column_default", [table, column, default]),
-              remove_command: command_str("remove_column", [table, column]),
-              code: backfill_code(table, column, default),
-              append: append
+              command: command_str("add_column_safely", [table, column, type, options])
           end
 
           if type.to_s == "json" && postgresql?
             raise_error :add_column_json
           end
         when :change_column
-          table, column, type = args
+          table, column, type, options = args
+          options ||= {}
 
           safe = false
+          found_column = connection.columns(table).find { |c| c.name.to_s == column.to_s }
+          raise StrongMigrations::Error, "Column '#{column}' of relation '#{table}' does not exist" unless found_column
+
           # assume Postgres 9.1+ since previous versions are EOL
           if postgresql? && type.to_s == "text"
-            found_column = connection.columns(table).find { |c| c.name.to_s == column.to_s }
-            safe = found_column && found_column.type == :string
+            safe = found_column.type == :string
+          end
+
+          unless safe
+            down_options = {}
+            options.each do |option, value|
+              if value != found_column.send(option)
+                down_options[option] = found_column.send(option)
+              end
+            end
+            previous_type = found_column.type
+
+            raise_error :change_column,
+              up_command: command_str("change_column_safely", [table, column, type, options]),
+              down_command: command_str("change_column_safely", [table, column, previous_type, down_options])
           end
-          raise_error :change_column unless safe
         when :create_table
           table, options = args
           options ||= {}
@@ -186,10 +192,6 @@ def set_timeouts
 
     private
 
-    def connection
-      @migration.connection
-    end
-
     def version
       @migration.version
     end
@@ -202,22 +204,6 @@ def version_safe?
       version && version <= StrongMigrations.start_after
     end
 
-    def postgresql?
-      %w(PostgreSQL PostGIS).include?(connection.adapter_name)
-    end
-
-    def postgresql_version
-      @postgresql_version ||= begin
-        target_version = StrongMigrations.target_postgresql_version
-        if target_version && defined?(Rails) && (Rails.env.development? || Rails.env.test?)
-          # we only need major version right now
-          target_version.to_i * 10000
-        else
-          connection.execute("SHOW server_version_num").first["server_version_num"].to_i
-        end
-      end
-    end
-
     def raise_error(message_key, header: nil, **vars)
       return unless StrongMigrations.check_enabled?(message_key, version: version)
 
@@ -238,7 +224,7 @@ def raise_error(message_key, header: nil, **vars)
 
     def constraint_str(statement, identifiers)
       # not all identifiers are tables, but this method of quoting should be fine
-      code = statement % identifiers.map { |v| connection.quote_table_name(v) }
+      code = quote_identifiers(statement, identifiers)
       "safety_assured do\n      execute '#{code}' \n    end"
     end