Popular mistakes when using nested forms

Here are some popular mistakes when using nested forms:

• You are using fields_for instead of form.fields_for.
• You forgot to use accepts_nested_attributes in the containing model. Rails won't complain, but nothing will work. In particular, nested_form.object will be nil.
• The :reject_if option lambda in your accepts_nested_attributes call is defined incorrectly. Raise the attributes hash given to your :reject_if lambda to see if it looks like you expect.
• If you are nesting forms into nested forms, each model involved needs accepts_nested_attributes with a correct :reject_if lambda.
• You are iterating over associated records inside a form.fields_for block. Don't do that, fields_for already iterates for you.
• You used form.fields_for |nested_form|, but continue to call form helpers (like text_field) on the containing form instead of nested_form.
• When you have a has_many :through association, the nested form should be on the join model itself, instead of the model associated through the join model. _destroy checkboxes should probably be on the form for the join model, or you will end up with orphaned join model records.
• Nested records are validating the presence of their nesting parent record's foreign key. If you do this, you cannot create a new parent record together with a new child record and will need to save the parent before you can save the child. You can opt to only show the nested child form when the parent has been saved before, or simply let go of the validation.
• Your controller is slicing away the nested parameters.
• You are not building the right amount of associated records in the required controller actions with @model.nested_model.build.
  • If you are adding fields dynamically by using JavaScript, then the form helper method should take care of this: new_nested_record = f.object.send(nested_association).klass.new.
• You have not added the id attribute into the nested params: params.permit(:attribute_1, ..., nested_attributes: [:id, ...]). This will render additional records from the last cached response in form round trips, since the id can not be assigned by the server while it may be outputting Unpermitted parameter: :id within the console.
• You are not setting the inverse_of for a has_many through association. Rails will then not be able to process a collection assignment Show archive.org snapshot , since it can't find the inverse association.
• You are encountering a bug in Rails 2.3.8+ where nested attribute changes disappear.