Avoid using nested attributes in model methods
Implement params processing: KnownIPsController
Do not validate presence of Payment#payable_type

Do not validate presence of Payment#payable_type because, at least in Rails 4, it fails automatic saving of associated models, and its values should be set automatically anyway.
Clean up before upgrading to Rails 4.0
Use shorter db index names

Use db index names of at most 62 characters because the maximal allowed
length has decreased by 2 in Rails 4.
Upgrade to Rails 4.0 -- initial phase
Upgrade to Rails 4.0 WIP
Use association names instead of SQL in 'joins'

Use association name symbols instead of string SQL in 'joins' for defining scopes.  I have not debugged it, but it seems Rails 4.0 stopped working in some cases with SQL strings in 'joins' method.
Use 'includes' for LEFT JOIN for Rails 4

Rails 4 does not seem to accept string SQL conditions for 'joins' method.  Use 'includes' with the new API.
Adjust custom form builder to Rails 4 - WIP