Mongo DB查询公式

Question

I have a database that has the following collection:

{id1: "1", id2: "a"}, {id1: "2", id2: "a"}, {id1: "3", id2: "a"}, {id1: "1", id2: "b"}, {id1: "2", id2: "b"}

I want to find id2 given the list of id1 : 1,2

the answer given should be b given that it is the only id2 that has ONLY 1,2

If the list of 'id1' given were: 1,2,3 then the 'id2' found should be 'a' given that 'a' is associated with 1,2 AND 3

How do I query this on Mongodb preferably using aggregation.

Answer 1

I think I understand what you're trying to do. You can do it with an aggregate query, but it isn't very straight forward:

db.<collection_name>.aggregate([
    {$group:{_id:"$id2",id1Set:{$addToSet:"$id1"}}},
    {$match:{"id1Set":{$size:2,$all:["1","2"]}}}
])

This query will group the documents by id2 and create a set containing the values of id1. It then matches the documents from the group, looking at the size of the id1Set (should be 2) and the values in the id1Set should contain all of "1" and "2".

Since your example data contains string values for id2 ("1" and "2", rather than 1 and 2) the match is on string values rather than numbers.