$lookup with ObjectID $let variable

[Steve Jerman]

Hi,

Using Mongo 3.6.5

I am using $lookup in an aggregation pipeline. Can't get this to work:

db.getCollection('sensor_data').aggregate(

[

   ....

   {$lookup:{

        from:'sites',

        let: { z: '$zone'},

        pipeline: [

             {$match: { _id : '$$z'}},           

        ],

        as: 'apInfo'

     }},

]

$zone is an ObjectID in the sensor_data document. 

If, I do this:

db.getCollection('sensor_data').aggregate(

[

   ....

   {$lookup:{

        from:'sites',

        let: { z: '$zone'},

        pipeline: [

             {$project: { t : '$$z'}},           

        ],

        as: 'apInfo'

     }},

]

I get what you would expect ... apInfo is an array with _id being the ObjectID and t being the zone...

{

    "_id" : {

        "mac" : NumberLong(260515773225939),

        "ts" : ISODate("2018-07-07T16:32:49.000Z")

    },

    "zone" : ObjectId("5af1b83c1c2d750008146c3f"),

    "apInfo" : [ 

        {

            "_id" : ObjectId("5ac3f91f3314940008ab61d3"),

            "t" : ObjectId("5af1b83c1c2d750008146c3f")

        },

        {

            "_id" : ObjectId("5af1b83c1c2d750008146c3f"),

            "t" : ObjectId("5af1b83c1c2d750008146c3f")

        },  ...

Any suggestions?

Steve

[Steve Jerman]

OK, just answered my question..

This works:

             {$match: {   $expr : { $eq: [ "$$z", "$_id"] } }},

not sure why the simple version doesn't