Make BelongsTo fields nullable

[seanpdoyle]

Prior to this commit, there was no way to set a belongs_to
relationship to nil through the UI.

[c-lliope]

Good idea.

For the implementation, what do you think about leaving associated_resource_options, and changing candidate_resources to:

  def candidate_resources
    if options[:allow_nil]
      [nil] + associated_class.all
    else
      associated_class.all
    end
  end

Also, can you add a test to spec/lib/fields/has_many_spec.rb?

[seanpdoyle]

@Graysonwright is this related to Fields::HasMany?

[seanpdoyle]

@Graysonwright do we need an allow_nil option?

If we assume that all BelongsTo relations are nullable, the API would stay small and focussed, and any validation failures would be surfaced like normal (through form_for integration with ActiveModel validations)

[c-lliope]

Yeah, good point.

I'm 👍 on this - merge away.

As for how Field::BelongsTo is related to Field::HasMany, they share the same relationship as the ActiveRecord relationships - they can be used independently of one another, but they're usually used as inverses of each other. They both inherit from Administrate::Field::Associative.