Rails monkey patch only on certain columns - ruby-on-rails

Question

I am trying to include a module only if the subclass has a certain column,
and I am doing this in an initializer:
class ActiveRecord::Base
def self.inherited(subclass)
subclass.include(MultiTenancy) if subclass.new.respond_to?(:tenant_id)
end
end
I keep getting this error:
NoMethodError: undefined method `[]' for nil:NilClass
Here is the module that I am importing:
module MultiTenancy
class TenantNotSetError < StandardError ; end
def self.included(model)
model.class_eval do
belongs_to :tenant
validates :tenant_id, presence: true
default_scope -> {
raise TenantNotSetError.new unless Tenant.current_tenant
where(tenant_id: Tenant.current_tenant.id)
}
def multi_tenanted?
true
end
end
end
end
What am I doing wrong. I can include the module on any subclass separate eg. Klass.include(MultiTenancy) successfully, so the problem is with the initializer.
Below is the stack trace:
from -e:1:in `<main>'2.1.1 :002 > Klass.all
NoMethodError: undefined method `[]' for nil:NilClass
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activerecord-4.1.4/lib/active_record/relation/delegation.rb:9:in `relation_delegate_class'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activerecord-4.1.4/lib/active_record/relation/delegation.rb:112:in `relation_class_for'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activerecord-4.1.4/lib/active_record/relation/delegation.rb:106:in `create'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activerecord-4.1.4/lib/active_record/model_schema.rb:133:in `table_name='
from /home/lee/Code/mobifit/app/models/klass.rb:25:in `<class:Klass>'
from /home/lee/Code/mobifit/app/models/klass.rb:22:in `<top (required)>'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:443:in `load'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:443:in `block in load_file'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:633:in `new_constants_in'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:442:in `load_file'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:342:in `require_or_load'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:480:in `load_missing_constant'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:180:in `const_missing'
from (irb):2
from /home/lee/.rvm/gems/ruby-2.1.1/gems/railties-4.1.4/lib/rails/commands/console.rb:90:in `start'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/railties-4.1.4/lib/rails/commands/console.rb:9:in `start'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/railties-4.1.4/lib/rails/commands/commands_tasks.rb:69:in `console'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/railties-4.1.4/lib/rails/commands/commands_tasks.rb:40:in `run_command!'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/railties-4.1.4/lib/rails/commands.rb:17:in `<top (required)>'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:247:in `require'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:247:in `block in require'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:232:in `load_dependency'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:247:in `require'
from /home/lee/Code/mobifit/bin/rails:8:in `<top (required)>'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:241:in `load'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:241:in `block in load'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:232:in `load_dependency'
from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:241:in `load'
from /home/lee/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /home/lee/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from -e:1:in `<main>'2.1.1 :003 >

Answer 1

If you follow the stack trace back a bit, you can see that the thing that's nil that isn't supposed to be is is the class's #relation_delegate_cache. Looking at the source, you can see that this normally gets initialized in the initialize_relation_delegate_cache method, which is called in the inherited hook for ActiveRecord::Delegation::DelegateCache.
ActiveRecord::Base extends this module, so normally a class that inherits from ActiveRecord::Base would call this inherited hook. So the problem with your code is that you're monkey-patching inherited on ActiveRecord::Base, which means that the hook in ActiveRecord::Delegation::DelegateCache is no longer being called.
Two comments:
This is why it's a bad idea to monkey-patch, especially something as fundamental and complex as active record. Is it possible to do what you want to do in a more straightforward way?
If it's not, you should take some notes on how ActiveRecord::Delegation::DelegateCache works and rewrite your monkey patch to mix in a module of your own, rather than just over-writing ActiveRecord::Base's inherited method.
The suggestion in (2) would look something like this:
module MultitenancyConcern
def self.inherited(subclass)
subclass.include(MultiTenancy) if subclass.column_names.include?("tenant_id")
super #this is critical for avoiding the error you're getting
end
end
then your monkey-patch becomes just
ActiveRecord::Base.extend(MultitenancyConcern)