View Source Ash.Query.Parent (ash v3.1.7)

true if the provided field is nil Used to access values from the "source" of a given expression.

This is used in cases where expressions are given for some relationship path, for example:any()

 has_many :foo, Foo do
   filter expr(priority == :foo and type == parent(foo_type))

This is supported on a case by case basis by a given data layer and in specific usages.