Added ability to use modules in searchable method

[LaurensN]

This PR gives you the ability to reference Models from Rails Engines / Modules.

For example:

def searchable_columns
# list columns inside the Array in string dot notation.
# Example: 'users.email'
@searchable_columns ||= [
"module_or_engine_name/model_name.column_name",
]
end

The sortable method seems to work fine using the normal notation (module_or_engine_name_model_name.column_name)

[antillas21]

@LaurensN could you share a test for this code?

[antillas21]

@LaurensN it's been a while since this PR has been open... we've finally come to grips on how we would like to define searchable and sortable columns. Please take a look at our proposed changes for this feature here: https://github.com/antillas21/ajax-datatables-rails/tree/version-0-3-0#searchable-and-sortable-columns-syntax

The methods that make this possible:

• searchable columns
• sortable columns

We would like to hear any feedback you can provide 😄