CODEWITHSUNDEEP
arraysmongodbmongodb-query
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adpap

Reputation: 209

Find document with array that consists exclusively particular documents in MongoDB

I have collection path_test with 2 documents in it

Document 1

{
  "_id" : 1,
  "tpc" : 5,
  "path" : [ 
    {
      "nids" : [ 0, 10, 11 ],
      "ctc" : 2
    }, 
    {
      "nids" : [ 0, 10 ],
      "ctc" : 2
    }, 
    {
      "nids" : [ 0, 10, 21 ],
      "ctc" : 1
    }
  ]
}

Document 2

{
  "_id" : 2,
  "tpc" : 5,
  "path" : [ 
    {
      "nids" : [ 0, 10, 110 ],
      "ctc" : 1
    }, 
    {
      "nids" : [ 0, 10, 11 ],
       "ctc" : 2
    }, 
    {
      "nids" : [ 0, 5 ],
      "ctc" : 2
    }
  ]
}

What I'm trying to get as a result are documents with path array in which all elements have nids like [0, 10, *]. Order is important, so [10, 0, *] will be wrong. It should find Document 1, but not Document 2. Was hoping I can resolve this with a query, before I start using map-reduce or aggregation.

This is what I've tried so far

Query1

db.getCollection('path_test').find( {
  "path": { $not: { $elemMatch: { "nids.0": { $nin: [0] }, "nids.1": { $nin: [10] } } } }
});

Query 2

db.getCollection('path_test').find( {
  "path.nids": { $not: { $elemMatch: { $nin: [0, 10] } } }
});

but both queries give me results where only 0 is in or where only 10 is in, but I need both and in that exact order.

Is that possible?

Upvotes: 3

Views: 353

Answers (1)

Mr Tarsa
Mr Tarsa

Reputation: 6652

not at least one means noone

Query 1

For simplification, lets assign

A = "nids.0": { $ne: 0 }
B = "nids.1": { $ne: 10 }
C = { A, B }

then

{ "path" : { $elemMatch: C } }

will find documents where at least one element in path array satisfies condition C, while

{ "path" : { $not: { $elemMatch: C } } }

will find documents where there are no element in path array that satisfies condition C.

Document 1 and Document 2 don't have elements in their path arrays that satisfy condition C, thus the Query1 output contains both of them. If, f.e, you add to the path array of the Document 1

{ "nids": [ 1, 11, 110], "ctc" : 1 }

then Document 1 will not be in the output of Query 1 becase this added element satisfies C.

Query 2

For simplification, lets assign

C = { $nin: [0, 10] }

then

{ "path.nids" : { $not: { $elemMatch: C } } }

will find documents where there are no element in path.nids array that satisfies condition C.

Document 1 and Document 2 in their path.nids arrays have elements that satisfy condition C, thus the Query 2 output contains neither of them. If, f.e, you add to you collection document

{ "_id" : 6, "tpc" : 5, "path" : [ { "nids" : [ 0, 10 ], "ctc" : 1 } ] }

then it will be in the output of Query 2 because in path.nids array there are no elements that satisfy C.

Solution

In Query 1 replace

{ $elemMatch: { "nids.0": { $nin: [0] }, "nids.1": { $nin: [10] } } }

with

{ $elemMatch: { $or: [ { "nids.0": { $ne: 0 } }, { "nids.1": { $ne: 10 } } ] } }

This new Query will find documents where there are no element in path array that satisfies at least one of conditions A and B. So, it will find Document 1, but not Document 2 (where "nids" : [ 0, 5 ] does not satisfy condition B.

Note that { $ne: 10 } is equivalent to { $nin: [10] }.

Upvotes: 2

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