Mongo DB - Second Level Search - elemMatch - mongodb

Question

I am trying to fetch all records (and count of all records) for a structure like the following,
{
id: 1,
level1: {
level2:
[
{
field1:value1;
},
{
field1:value1;
},
]
}
},
{
id: 2,
level1: {
level2:
[
{
field1:null;
},
{
field1:value1;
},
]
}
}
My requirement is to fetch the number of records that have field1 populated (atleast one in level2). I need to say fetch all the ids or the number of such ids.
The query I am using is,
db.table.find({},
{
_id = id,
value: {
$elemMatch: {'level1.level2.field1':{$exists: true}}
}
}
})
Please suggest.
EDIT1:
This is the question I was trying to ask in the comment. I was unable to elucidate in the comment properly. Hence, editing the question.
{
id: 1,
level1: {
level2:
[
{
field1:value1;
},
{
field1:value1;
},
]
}
},
{
id: 2,
level1: {
level2:
[
{
field1:value2;
},
{
field1:value2;
},
{
field1:value2;
}
]
}
}
{
id: 3,
level1: {
level2:
[
{
field1:value1;
},
{
field1:value1;
},
]
}
}
The query we used results in
value1: 4
value2: 3
I want something like
value1: 2 // Once each for documents 1 & 3
value2: 1 // Once for document 2

Answer 1

You can do that with the following find query:
db.table.find({ "level1.level2" : { $elemMatch: { field1 : {$exists: true} } } }, {})
This will return all documents that have a field1 in the "level1.level2" structure.
For your question in the comment, you can use the following aggregation to "I had to return a grouping (and the corresponding count) for the values in field1":
db.table.aggregate(
[
{
$unwind: "$level1.level2"
},
{
$match: { "level1.level2.field1" : { $exists: true } }
},
{
$group: {
_id : "$level1.level2.field1",
count : {$sum : 1}
}
}
]
UPDATE: For your question "'value1 - 2` At level2, for a document, assume all values will be the same for field1.".
I hope i understand your question correctly, instead of grouping only on the value of field1, i added the document _id as an xtra grouping:
db.table.aggregate(
[
{
$unwind: "$level1.level2"
},
{
$match: {
"level1.level2.field1" : { $exists: true }
}
},
{
$group: {
_id : { id : "$_id", field1: "$level1.level2.field1" },
count : {$sum : 1}
}
}
]
);
UPDATE2:
I altered the aggregation and added a extra grouping, the aggregation below gives you the results you want.
db.table.aggregate(
[
{
$unwind: "$level1.level2"
},
{
$match: {
"level1.level2.field1" : { $exists: true }
}
},
{
$group: {
_id : { id : "$_id", field1: "$level1.level2.field1" }
}
},
{
$group: {
_id : { id : "$_id.field1"},
count : { $sum : 1}
}
}
]
);