Add STI relationship option

[lgebhardt]

This fixes a few issues I found with STI

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[bf4]

I think this is what @JamesGlover was referring to

[bf4]

Suggested change

	pluck_fields << Arel.sql("#{concat_table_field(linkage_table_alias, 'type')} AS \"#{linkage_table_alias}_type\"")
	pluck_fields << Arel.sql("#{concat_table_field(linkage_table_alias, klass.inheritance_column)} AS \"#{linkage_table_alias}_type\"")

right?

[JamesGlover]

I think at this point klass is the resource, not the ActiveRecord::Base class, so you'd also need to define an inheritance_column method on JSONAPI::BasicResource.

Although in practice you'd probably want to use _inheritance_column instead, to be consistent with _primary_key.

# lib/jsonapi/basic_resource.rb
def inheritance_column(key)
  @_inheritance_column = key.to_sym
end

def _inheritance_column
  @_inheritance_column ||= _default_inheritance_column
end

def _default_inheritance_column
  @_default_inheritance_column ||=_model_class.respond_to?(:inheritance_column) ? _model_class.inheritance_column : :type
end

[bf4]

Suggested change

	pluck_fields << Arel.sql("#{concat_table_field(resource_table_alias, 'type')} AS #{resource_table_alias}_type")
	pluck_fields << Arel.sql("#{concat_table_field(resource_table_alias, klass.inheritance_column)} AS #{resource_table_alias}_type")

right?