Show SQL generated by pending migrations in rails without updating the database - ruby-on-rails

Question

I'd like to have a way to produce the actual sql (ie: if I pasted into a mysql console, it would work) that will be generated by a rake db:migrate without actually updating the target database.
rake db:migrate:status does a good job of showing which migrations are pending for a given database, but I've yet to find a way to get the actual SQL produced.
Any ideas?

Answer 1

Very interesting question! I found this way:
Assume your migration placed in file db/migrate/20160102210050_create_items.rb and called CreateItems
Go to Rails console and load migration file:
rails c
require './db/migrate/20160102210050_create_items'
Open transaction, run migration and rollback transaction before commit :)
ActiveRecord::Base.connection.transaction do
CreateItems.new.migrate :up
raise ActiveRecord::Rollback
end
If you want to check SQL on rollback, just call CreateItems.new.migrate :down on step 3. SQL will be executed and tested on database, but not committed - so you can verify your migration without affects.

Answer 2

This can be done by monkey-patching the database adapter. This example works for MySQL.
Create a rake task for "fake db:migrate":
desc "Prints all SQL to be executed during pending migrations"
task :fake_db_migrate => :environment do
module ActiveRecord
module ConnectionAdapters
class AbstractMysqlAdapter < AbstractAdapter
alias_method :real_execute, :execute
def execute(sql, name = nil)
if sql =~ /^SHOW/ || sql =~ /^SELECT.*FROM.*schema_migrations/ || sql =~ /^SELECT.*information_schema/m
real_execute(sql, name)
else
puts sql
end
end
end
end
end
Rake::Task["db:migrate"].invoke
end
The rake task monkey-patches the execute method in the connection adapter so that SQL is printed instead of being executed, before actually running the migrations. However, we still have to execute some of the internal SQLs that are used by the db:migrate task to get the database schema and to find out which migrations are pending. That's what the real_execute call does.
Test
Suppose now that and we have a pending migration in db/migrate/20160211212415_create_some_table.rb:
class CreateSomeTable < ActiveRecord::Migration
def change
create_table :some_table do |t|
t.string :string_column, null: false, default: 'ok'
t.timestamps
end
end
end
$ rake db:migrate:status
...
down 20160211212415 Create some table
Now, let's run our fake migrations task:
$ rake fake_db_migrate
== 20160211212415 CreateSomeTable: migrating ==================================
-- create_table(:some_table)
CREATE TABLE `some_table` (`id` int(11) auto_increment PRIMARY KEY, `string_column` varchar(255) DEFAULT 'ok' NOT NULL, `created_at` datetime, `updated_at` datetime) ENGINE=InnoDB
-> 0.0009s
== 20160211212415 CreateSomeTable: migrated (0.0010s) =========================
BEGIN
INSERT INTO `schema_migrations` (`version`) VALUES ('20160211212415')
COMMIT
The migrations status has not been changed, i.e. the migration is still pending:
$ rake db:migrate:status
...
down 20160211212415 Create some table
Tested on rails 4.2.3 with the mysql2 gem.

Answer 3

A slightly more low-level function, which can be used for your purpose:
# Get SQL query of a migration expression without executing it.
#
# #example
# schema_statement_to_sql { create_table(:tomatoes) }
# # => "CREATE TABLE \"tomatoes\" (\"id\" serial primary key) "
def schema_statement_to_sql(&block)
raise ArgumentError, 'No block given' unless block_given?
connection = ActiveRecord::Base.connection
original_execute = connection.method(:execute)
sql_to_return = ''
capturing_execute = proc { |sql| sql_to_return = sql }
connection.define_singleton_method(:execute, &capturing_execute)
begin
connection.instance_eval(&block)
ensure
connection.define_singleton_method(:execute, &original_execute)
end
sql_to_return
end

Answer 4

You can do
rake db:migrate --dry-run --trace
and rake will test the task. Then use one of the methods listed to get the SQL that would be run.