Add filters_association that allow specification of multiple parts within an aggregation

[elrayle]

Reference: https://github.com/projecthydra-labs/hydra-pcdm/wiki/Proposed-Interfaces#defining-an-aggregation-with-multiple-filters

Example definition:

aggregates :members, predicate: RDFVocabularies::PCDMTerms.hasMember,
           class_name: "ActiveFedora::Base"

filters_association :members, as: :child_objects, condition: :pcdm_object?
filters_association :members, as: :child_collections, condition: :pcdm_collection?

By defining a filters_association, you get the following methods defined that provide getters/setters with the method name in as: and validation using the method specified in condition:.

child_objects=    # validate with pcdm_object?
child_objects+=   # validate with pcdm_object?
child_objects<<   # validate with pcdm_object?
child_objects     # filter to return only objects

child_collections=    # validate with pcdm_collection?
child_collections+=   # validate with pcdm_collection?
child_collections<<   # validate with pcdm_collection?
child_collections     # filter to return only collections

members=    # validate with pcdm_object and pcdm_collection?
members+=   # validate with pcdm_object and pcdm_collection?
members<<   # validate with pcdm_object and pcdm_collection?
members     # return everything in the aggregation

[elrayle]

Deleting objects also needs to be taken into consideration.

child_objects.delete obj              # delete first occurrence of obj
child_objects.delete obj, nth         # delete nth occurrence of obj relative to the filtered list of child_objects

child_collections.delete col          # delete first occurrence of col
child_collections.delete col, nth     # delete nth occurrence of col relative to the filtered list of child_collections

members.delete obj          # delete first occurrence of obj
members.delete obj, nth     # delete nth occurrence of obj relative to all members
members.delete col          # delete first occurrence of col
members.delete obj, nth     # delete nth occurrence of col relative to all members

See also #46