docs: function extractFKs(schema) {
  	var FKs = {};
  
  	_.forEach(schema.paths, function(path, id) {
  		if (id == 'id' || id == '_id') {
  			// Pass
  		} else if (path.instance && path.instance == 'ObjectID') {
  			FKs[id] = {type: FK_OBJECTID};
  		} else if (path.caster && path.caster.instance == 'ObjectID') { // Array of ObjectIDs
  			FKs[id] = {type: FK_OBJECTID_ARRAY};
  		} else if (path.schema) {
  			FKs[id] = {
  				type: FK_SUBDOC,
  				fks: extractFKs(path.schema),
  			};
  		}
  	});
  
  	return FKs;
  }