MongoDB - filtering an array based on another array

Question

I'm using MongoDB and this is what my documents look like

{
    "_id": 1,
   "arr1": ["a", "a", "b", "c"],
   "arr2": [1, 4, 2, 3 ],
},
{
    "_id": 2,
   "arr1": ["z", "a", "b", "a"],
   "arr2": [1, 4, 2, 3 ],
}

The arrays in keys arr1 and arr2 always have the same length. I would like to aggregate the data such that can I filter out the values in arr2 based on arr1.

For example, if I filter my collection based on arr1 looking for a, I would like to get something like that:

{ "_id": 1, "arr1": "a", "arr2": 1},
{ "_id": 1, "arr1": "a", "arr2": 4 },
{ "_id": 2, "arr1": "a", "arr2": 3 },
{ "_id": 2, "arr1": "a", "arr2": 3 }

Is there some method to accomplish that using MongoDB?

Thanks,

Uirá

Answer 1

Try this:

db.col1.aggregate([
   {  $unwind: 
         { path:"$arr2", includeArrayIndex: "i"}},
   {  $project: 
         {
           "arr2":1,
           "arr1": { $slice: ["$arr1","$i",1] }}
          },
   {$unwind: "$arr1"}
   {$match: { "arr1":"a" }}
 ])

Output from first $unwind Stage

{ "_id" : 1, "arr1" : [ "a", "a", "b", "c" ], "arr2" : 1, "i" : NumberLong(0) }
{ "_id" : 1, "arr1" : [ "a", "a", "b", "c" ], "arr2" : 4, "i" : NumberLong(1) }
{ "_id" : 1, "arr1" : [ "a", "a", "b", "c" ], "arr2" : 2, "i" : NumberLong(2) }
{ "_id" : 1, "arr1" : [ "a", "a", "b", "c" ], "arr2" : 3, "i" : NumberLong(3) }
{ "_id" : 2, "arr1" : [ "z", "a", "b", "a" ], "arr2" : 1, "i" : NumberLong(0) }
{ "_id" : 2, "arr1" : [ "z", "a", "b", "a" ], "arr2" : 4, "i" : NumberLong(1) }
{ "_id" : 2, "arr1" : [ "z", "a", "b", "a" ], "arr2" : 2, "i" : NumberLong(2) }
{ "_id" : 2, "arr1" : [ "z", "a", "b", "a" ], "arr2" : 3, "i" : NumberLong(3) }

Out put after second project stage

{ "_id" : 1, "arr1" : [ "a" ], "arr2" : 1 }
{ "_id" : 1, "arr1" : [ "a" ], "arr2" : 4 }
{ "_id" : 1, "arr1" : [ "b" ], "arr2" : 2 }
{ "_id" : 1, "arr1" : [ "c" ], "arr2" : 3 }
{ "_id" : 2, "arr1" : [ "z" ], "arr2" : 1 }
{ "_id" : 2, "arr1" : [ "a" ], "arr2" : 4 }
{ "_id" : 2, "arr1" : [ "b" ], "arr2" : 2 }
{ "_id" : 2, "arr1" : [ "a" ], "arr2" : 3 }

second $unwind Output

{ "_id" : 1, "arr1" : "a", "arr2" : 1 }
{ "_id" : 1, "arr1" : "a", "arr2" : 4 }
{ "_id" : 1, "arr1" : "b", "arr2" : 2 }
{ "_id" : 1, "arr1" : "c", "arr2" : 3 }
{ "_id" : 2, "arr1" : "z", "arr2" : 1 }
{ "_id" : 2, "arr1" : "a", "arr2" : 4 }
{ "_id" : 2, "arr1" : "b", "arr2" : 2 }
{ "_id" : 2, "arr1" : "a", "arr2" : 3 }

Final $match output:

{ "_id" : 1, "arr1" : [ "a" ], "arr2" : 1 }
{ "_id" : 1, "arr1" : [ "a" ], "arr2" : 4 }
{ "_id" : 2, "arr1" : [ "a" ], "arr2" : 4 }
{ "_id" : 2, "arr1" : [ "a" ], "arr2" : 3 }