There was a silly error in my controller code!
Method was this:
def filter
@user = User.find(params[:user_id])
respond_to do |format|
format.html
format.json { render json: @user }
end
end
Changed it to this:
def filter
@user = User.find(params[:user_id])
@user_notes = UserNote.find_by_user_id(@user)
end