Distinct array element with condition - mongodb

Question

My documents look like this:
{
"_id": "1",
"tags": [
{ "code": "01-01", "type": "machine" },
{ "code": "04-06", "type": "gearbox" },
{ "code": "07-01", "type": "machine" }
]
},
{
"_id": "2",
"tags": [
{ "code": "03-04","type": "gearbox" },
{ "code": "01-01", "type": "machine" },
{ "code": "04-11", "type": "machine" }
]
}
I want to get distinct codes only for tags whose type is "machine". so, for the example above, the result should be ["01-01", "07-01", "04-11"].
How do I do this?

Answer 1

Using $unwind and then $group with the tag as the key will give you each tag in a separate document in your result set:
db.collection_name.aggregate([
{
$unwind: "$tags"
},
{
$match: {
"tags.type": "machine"
}
},
{
$group: {
_id: "$tags.code"
}
},
{
$project:{
_id:false
code: "$_id"
}
}
]);
Or, if you want them put into an array within a single document, you can use $push within a second $group stage:
db.collection_name.aggregate([
{
$unwind: "$tags"
},
{
$match: {
"tags.type": "machine"
}
},
{
$group: {
_id: "$tags.code"
}
},
{
$group:{
_id: null,
codes: {$push: "$_id"}
}
}
]);
Another user suggested including an initial stage of { $match: { "tags.type": "machine" } }. This is a good idea if your data is likely to contain a significant number of documents that do not include "machine" tags. That way you will eliminate unnecessary processing of those documents. Your pipeline would look like this:
db.collection_name.aggregate([
{
$match: {
"tags.type": "machine"
}
},
{
$unwind: "$tags"
},
{
$match: {
"tags.type": "machine"
}
},
{
$group: {
_id: "$tags.code"
}
},
{
$group:{
_id: null,
codes: {$push: "$_id"}
}
}
]);

Answer 2

> db.foo.aggregate( [
... { $unwind : "$tags" },
... { $match : { "tags.type" : "machine" } },
... { $group : { "_id" : "$tags.code" } },
... { $group : { _id : null , "codes" : {$push : "$_id"} }}
... ] )
{ "_id" : null, "codes" : [ "04-11", "07-01", "01-01" ] }

Answer 3

A better way would be to group directly on tags.type and use addToSet on tags.code.
Here's how we can achieve the same output in 3 stages of aggregation :
db.name.aggregate([
{$unwind:"$tags"},
{$match:{"tags.type":"machine"}},
{$group:{_id:"$tags.type","codes":{$addToSet:"$tags.code"}}}
])
Output : { "_id" : "machine", "codes" : [ "04-11", "07-01", "01-01" ] }
Also, if you wish to filter out tag.type codes, we just need to replace "machine" in match stage with desired tag.type.