Be careful when using params.merge as params is a HashWithIndifferentAccess Show archive.org snapshot .

Why?

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Usually this should not be an issue but it turns crazy if you try to include associated models deeper than 1 level:
options = params.merge(:include => { :user => :avatar })
Post.paginate options

When inspecting the merged params you will get something like this:
{ :include=> { "user" => :avatar }, :page => 23 }

Here the :user symbol in the hash of inclusions turned into a "user" string.\
This breaks Rails' loading of associations as it will complain about calling nil.name or nil.macro while it tries to process the association list.

Best practice

Make sure to do it this way:
params.to_hash.symbolize_keys.merge(:include => { :user => :avatar })

(Note that params.to_hash returns a hash with string keys.)