fields_for form builder object is nil

Question

Any way to access a nested form_bulder.object?

## controller
@project = Project.new
@project.tasks.build

form_for(@project) do |f|
  f.object.nil? ## returns false

  fields_for :tasks do |builder|
    builder.object.nil? ## returns true
  end
end

Answer 1

You must have accepts_nested_attributes_for in the Project model in order for the object to be passed.

class Project < ActiveRecord::Base
  has_many :tasks
  accepts_nested_attributes_for :tasks ## this is required
end

Answer 2

fields_for requires that the method tasks_attributes= exists. accepts_nested_attributes_for :tasks creates this method for you, but you can also just define it yourself:

def tasks_attributes=(params)
  # ... manually apply attributes in params to tasks
end

When this method does not exist, the builder.object ends up being nil.