MongoDb aggregation query with $group and $push in

问题:

I have a question regarding the $group argument of MongoDb aggregations. My data structure looks as follows:

My "Event" collection contains this single document:

{
   "_id": ObjectId("mongodbobjectid..."),
   "name": "Some Event",
   "attendeeContainer": {
       "min": 0,
       "max": 10,
       "attendees": [
            {
               "type": 1,
               "status": 2,
               "contact": ObjectId("mongodbobjectidHEX1")
            },
            {
               "type": 7,
               "status": 4,
               "contact": ObjectId("mongodbobjectidHEX2")
            }
        ]
    }
}

My "Contact" collection contains these documents:

{
    "_id": ObjectId("mongodbobjectidHEX1"),
    "name": "John Doe",
    "age": 35
},
{
    "_id": ObjectId("mongodbobjectidHEX2"),
    "name": "Peter Pan",
    "age": 60
}

What I want to do is perform an aggregate query on the "Event" collection and get the following result with full "contact" data:

{
   "_id": ObjectId("mongodbobjectid..."),
   "name": "Some Event",
   "attendeeContainer": {
       "min": 0, 
       "max": 10,
       "attendees": [
             {
                "type": 1,
                "status": 2,
                "contact": {
                   "_id": ObjectId("mongodbobjectidHEX1"),
                   "name": "John Doe",
                   "age": 35
                }
             },
             {
                "type": 7,
                "status": 4,
                "contact": {
                   "_id": ObjectId("mongodbobjectidHEX2"),
                   "name": "Peter Pan",
                   "age": 60
                }
             }
         ]
     }
}

The arguments I am using right now look as follows (shortened version):

"$unwind" : "$attendeeContainer.attendees",
"$lookup" : { "from" : "contactinfo", "localField" : "attendeeContainer.attendees.contact","foreignField" : "_id", "as" : "contactInfo" },
"$unwind" : "$contactInfo",
"$group"  : { "_id": "$_id", 
              "name": { "$first" : "$name" }, 
              ...
              "contact": { "$push": { "contact": "$contactInfo"} }
            }

However, this leads to the "contact" array being on "Event" level (because of the grouping) instead of one document of the array being at each "attendeeContainer.attendees". How can I push the "contact" array to be at "attendeeContainer.attendees"? (as shown in the desired output above)

I tried things like:

"attendeeContainer.attendees.contact": { "$push": { "contact": "$contactInfo"} }

But mongodb apparently does not allow "." at $group stage.

回答1:

Try running the following aggregation pipeline, the key is using a final $project pipeline to create the attendeeContainer subdocument:

db.event.aggregate([
    { "$unwind": "$attendeeContainer.attendees" },
    {
        "$lookup" : { 
            "from" : "contactinfo", 
            "localField" : "attendeeContainer.attendees.contact",
            "foreignField" : "_id", 
            "as" : "attendeeContainer.attendees.contactInfo" 
        }
    },
    { "$unwind": "$attendeeContainer.attendees.contactInfo" },
    {
        "$group": {
            "_id" : "$_id",
            "name": { "$first": "$name" },   
            "min" : { "$first": "$attendeeContainer.min" },
            "max" : { "$first": "$attendeeContainer.max" },
            "attendees": { "$push": "$attendeeContainer.attendees" }            
        }
    },
    {
        "$project": {
            "name": 1,
            "attendeeContainer.min": "$min",
            "attendeeContainer.max": "$min",
            "attendeeContainer.attendees": "$attendees"
        }
    }
])

---

Debugging Tips

Debugging the pipeline at the 4th stage, you would get the result

db.event.aggregate([
    { "$unwind": "$attendeeContainer.attendees" },
    {
        "$lookup" : { 
            "from" : "contactinfo", 
            "localField" : "attendeeContainer.attendees.contact",
            "foreignField" : "_id", 
            "as" : "attendeeContainer.attendees.contactInfo" 
        }
    },
    { "$unwind": "$attendeeContainer.attendees.contactInfo" },
    {
        "$group": {
            "_id": "$_id",
            "name": { "$first": "$name" },   
            "min" : { "$first": "$attendeeContainer.min" },
            "max" : { "$first": "$attendeeContainer.max" },
            "attendees": { "$push": "$attendeeContainer.attendees" }            
        }
    }/*,
    {
        "$project": {
            "name": 1,
            "attendeeContainer.min": "$min",
            "attendeeContainer.max": "$min",
            "attendeeContainer.attendees": "$attendees"
        }
    }*/
])

Pipeline result

{
    "_id" : ObjectId("582c789282a9183adc0b53f5"),
    "name" : "Some Event",
    "min" : 0,
    "max" : 10,
    "attendees" : [ 
        {
            "type" : 1,
            "status" : 2,
            "contact" : ObjectId("582c787682a9183adc0b53f3"),
            "contactInfo" : {
                "_id" : ObjectId("582c787682a9183adc0b53f3"),
                "name" : "John Doe",
                "age" : 35
            }
        }, 
        {
            "type" : 7,
            "status" : 4,
            "contact" : ObjectId("582c787682a9183adc0b53f4"),
            "contactInfo" : {
                "_id" : ObjectId("582c787682a9183adc0b53f4"),
                "name" : "Peter Pan",
                "age" : 60
            }
        }
    ]
}

and the final $project pipeline will give you the desired result:

db.event.aggregate([
    { "$unwind": "$attendeeContainer.attendees" },
    {
        "$lookup" : { 
            "from" : "contactinfo", 
            "localField" : "attendeeContainer.attendees.contact",
            "foreignField" : "_id", 
            "as" : "attendeeContainer.attendees.contactInfo" 
        }
    },
    { "$unwind": "$attendeeContainer.attendees.contactInfo" },
    {
        "$group": {
            "_id": "$_id",
            "name": { "$first": "$name" },   
            "min" : { "$first": "$attendeeContainer.min" },
            "max" : { "$first": "$attendeeContainer.max" },
            "attendees": { "$push": "$attendeeContainer.attendees" }            
        }
    },
    {
        "$project": {
            "name": 1,
            "attendeeContainer.min": "$min",
            "attendeeContainer.max": "$min",
            "attendeeContainer.attendees": "$attendees"
        }
    }/**/
])

Desired/Actual Output

{
    "_id" : ObjectId("582c789282a9183adc0b53f5"),
    "name" : "Some Event",
    "attendeeContainer" : {
        "min" : 0,
        "max" : 10,
        "attendees" : [ 
            {
                "type" : 1,
                "status" : 2,
                "contact" : ObjectId("582c787682a9183adc0b53f3"),
                "contactInfo" : {
                    "_id" : ObjectId("582c787682a9183adc0b53f3"),
                    "name" : "John Doe",
                    "age" : 35
                }
            }, 
            {
                "type" : 7,
                "status" : 4,
                "contact" : ObjectId("582c787682a9183adc0b53f4"),
                "contactInfo" : {
                    "_id" : ObjectId("582c787682a9183adc0b53f4"),
                    "name" : "Peter Pan",
                    "age" : 60
                }
            }
        ]
    }
}