Two alias_method_chain calls: first works second doesn't

rubyonrails-talk
1

In the example code in this Pastie Parked at Loopia, I'm observing behavior I can't explain. I'm making two calls to alias_method_chain to extend create_table and drop_table. The create_table alias chain works as expected. While debugging the drop_table_with alias chained method is never entered.

Thanks in advance for your help.

2

Any chance it's as simple as the placement of your requires?

3

I don't think so. Both create_table_with and drop_table_with are in the same file which is included from environment.rb's last line

ActiveRecord::ConnectionAdapters::SchemaStatements.send(:include, AuditingTables::ActiveRecord::ConnectionAdapters::SchemaStatements)

4

Okay,

I haven't dug deep into this but I can give you a few pointers.

First of all, the alias_method_chain is being done to a module so your methods only do stuff when they are actually included in a class.

With that in mind, what happens here?

module Failure   def failure_type     "everything is fine"   end

  def failure_type_with_foo     puts "I intercepted it!"     failure_type_without_foo   end   alias_method_chain :failure_type, :foo end

class SomethingThatFails   include Failure   def failure_type     "epic"   end end

SomethingThatFails.new.failure_type #=> "epic"

This could be an explanation.

Regards, Trevor

5

Sorry I should have been more specific. Your pastie shows:

line 13: def drop_table_with_auditing_table

line 20 require 'rails_generator' line 21 require 'rails_generator/scripts/generate'

line 23 def create_table_with_auditing_table

Since the code that works follows the requires at lines 20 and 21 I thought they might provide something helpful.

On a side note, depending on what your audit code needs to accomplish "acts_as_versioned" might be a good fit.

6

Sorry I should have been more specific. Your pastie shows:

line 13: def drop_table_with_auditing_table

line 20 require 'rails_generator' line 21 require 'rails_generator/scripts/generate'

line 23 def create_table_with_auditing_table

Since the code that works follows the requires at lines 20 and 21 I thought they might provide something helpful.

drop_table_with_auditing_table doesn't require the code in the requires. Also, that function's not even being entered.

7

Thanks for looking, but I believe I've tried what you suggest.

1) I believe the extension to the module is included in the extended module globally, because of the following line in environment.rb ActiveRecord::ConnectionAdapters::SchemaStatements.send(:include,AuditingTables::ActiveRecord::ConnectionAdapters::SchemaStatements)

2) I've used a debugger and seen that the failure_type_with_foo (my drop_table_with_auditing_table) in your example is never entered. However my create_table_with_auditing_table, which is defined and included in an identical manner is always entered.

Could something be overriding or blocking my extension of drop_table?

8

Yeah, that's the whole point of what I showed you. No amount of monkeypatching in the Failure module can compensate for the fact that the SomethingThatFails class defines its own failure_type() method.

Try to figure out the type of the object you are calling drop_table() on, then look at the rails source to see whether that class in question happens to define its own drop_table method.

If you're impatient you can always just search for "def drop_table" in the rails source.

Chances are, you're going to have to monkeypatch that class rather than SchemaStatements.

HTH Trevor