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How to filter a reference set efficiently in memory?

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Hi guys, I have a *-* (many to many) relation between City and Person ( person_city). One city has many persons and one person is associated to multiple cities. When I want to retrieve the matched persons  from the personlist (member: person_city) for a citylist (member: person_city) I DO NOT get the filtered person with that city, while I’d expect that. BUT when I do filter the reverse, filtering the persons  with the same city from the city list, I DO get the filtered the cities. I played further with it and it seems that the “filter” or “equal” list operation expects a corresponding subset of the reference set in order to filter it accordingly. So my question is, is there an efficient way for filtering persons based on a list of city, whereby I want to retrieve the persons associated to a city list with multiple cities.   Example; Person: A → Rotterdam, Den Haag B → Rotterdam, Amsterdam Personlist: A, B Citylist: Rotterdam, Den Haag , + 200 cities   Operation: filter personlist (A,B) with citylist (just Amsterdam). → Personlist (B) I’d expect that filter personlist/person_city with citylist (Amsterdam) would result in a personlist with just B, but unfortunately this is not the case (0 results) and I do not want to iterate over all the persons/cities to build a filtered list one by one.   there should be an efficient memory operation for this filter to get work.

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