rails - get the SQL of a model save - ruby-on-rails

Question

I need to be able to output the SQL UPDATES that would be generated by Rails, without actually running them or Saving the records. I will be outputting the SQL updates to a file instead.
Is there a way to do this in Rails, without using string interpolation?
Is it possible to do something like the following:
p = Post.where (something)
p.some_value = some_new_value
p.to_sql??? # how to generate the update statement
rather than:
"UPDATE TABLE SET field_1 = #{new_field} WHERE ID = " etc etc

Answer 1

Took from #R.F. Nelson and wrap it to a method. You could just calling to_update_sql with your model as the argument to get the SQL.
def to_update_sql(model)
return '' if model.changes.empty?
table = Arel::Table.new(model.class.table_name)
update_manager = Arel::UpdateManager.new(model.class)
update_manager.set(model.changes.map{|name, (_, val)| [table[name], val] })
.where(table[:id].eq(model.id))
.table(table)
return update_manager.to_sql
end
post = Post.first
post.some_value = xxxx
to_update_sql(post)
# => UPDATE `posts` SET `some_value` = xxx WHERE `posts`.`id` = 1

Answer 2

Taken from this post:
You can achieve this goal with AREL:
# Arel::InsertManager
table = Arel::Table.new(:users)
insert_manager = Arel::InsertManager.new
insert_manager.into(table)
insert_manager.insert([ [table[:first_name], 'Eddie'] ])
sql_transaction = insert_manager.to_sql
File.open('file_name.txt', 'w') do |file|
file.write(sql)
end
# Arel::UpdateManager
table = Arel::Table.new(:users)
update_manager = Arel::UpdateManager.new
update_manager.set([[table[:first_name], "Vedder"]]).where(table[:id].eq(1)).table(table)
sql_transaction = update_manager.to_sql
File.open('file_name.txt', 'w') do |file|
file.write(sql)
end
Here you can find all Arel managers, like delete_manager.rb, select_manager.rb and the others.
Good read: http://jpospisil.com/2014/06/16/the-definitive-guide-to-arel-the-sql-manager-for-ruby.html