Getting undefined method for ActiveRecord::Relation

Looks like @chapter is not a single Chapter object. If @chapter is initialized something like this:

@chapter = Chapter.where(:id => params[:id])

then you get a Relation object (that can be treated as a collection, but not a single object). So to fix this you need to retrieve a record using find_by_id, or take a first one from the collection

@chapter = Chapter.where(:id => params[:id]).first

or

@chapter = Chapter.find_by_id(params[:id])

As the others have said - adding the .first method will resolve this. I have experienced this issue when calling a @chapter by it's unique ID. Adding .first (or .take in Rails 4) will ensure only one object is returned.

Try: Chapter.find(params[:id]).first