Return specific array value field in aggregate - mongodb

Question

I have a issue in MongoDB i'm trying to build a very complex aggregate query, and its work almost as i want it, but i still have trobles, and the problems is i need to move a spefiect field so i can use it later.
My aggregate look like this right now.
db.getCollection('travel_sights').aggregate([{
'$match': {
'preview.photo' : {
'$exists':true
},
'_id': {
'$in' : [ObjectId("5b7af9701fbad410e10f32f7")]
}
}
},{
'$unwind' : '$preview.photo'
}, {
'$lookup':{
'from' : 'media_data',
'localField' : '_id',
'foreignField':'bind',
'as':'media'
}
}])
and it will return data like this.
{
"_id" : ObjectId("5b7af9701fbad410e10f32f7"),
"preview" : {
"photo" : {
"id" : ObjectId("5b7affea1fbad441494a663b"),
"sort" : 0
}
},
"media" : [
{
"_id" : ObjectId("5b7affea1fbad441494a663b")
},
{
"_id" : ObjectId("5b7b002d1fbad441494a663c")
},
{
"_id" : ObjectId("5b7b00351fbad441494a663d")
},
{
"_id" : ObjectId("5b7d9baa1fbad410de638bbb")
},
{
"_id" : ObjectId("5b7d9bae1fbad410e10f32f9")
},
{
"_id" : ObjectId("5b7d9bb11fbad441494a663e")
},
{
"_id" : ObjectId("5b7d9bb41fbad4ff97273402")
},
{
"_id" : ObjectId("5b7d9bb71fbad4ff99527e82")
},
{
"_id" : ObjectId("5b7d9bbb1fbad410de638bbc")
},
{
"_id" : ObjectId("5b7d9bbe1fbad410e10f32fa")
},
{
"_id" : ObjectId("5b7d9bc11fbad441494a663f")
},
{
"_id" : ObjectId("5b7d9bc41fbad4ff97273403")
},
{
"_id" : ObjectId("5b7d9bc71fbad4ff99527e83")
},
{
"_id" : ObjectId("5b7d9bca1fbad410de638bbd")
},
{
"_id" : ObjectId("5b7d9bcd1fbad441494a6640")
},
{
"_id" : ObjectId("5b7d9bd01fbad4ff97273404")
}
]
}
{
"_id" : ObjectId("5b7af9701fbad410e10f32f7"),
"preview" : {
"photo" : {
"id" : ObjectId("5b7b002d1fbad441494a663c"),
"sort" : 0
}
},
"media" : [
{
"_id" : ObjectId("5b7affea1fbad441494a663b")
},
{
"_id" : ObjectId("5b7b002d1fbad441494a663c")
},
{
"_id" : ObjectId("5b7b00351fbad441494a663d")
},
{
"_id" : ObjectId("5b7d9baa1fbad410de638bbb")
},
{
"_id" : ObjectId("5b7d9bae1fbad410e10f32f9")
},
{
"_id" : ObjectId("5b7d9bb11fbad441494a663e")
},
{
"_id" : ObjectId("5b7d9bb41fbad4ff97273402")
},
{
"_id" : ObjectId("5b7d9bb71fbad4ff99527e82")
},
{
"_id" : ObjectId("5b7d9bbb1fbad410de638bbc")
},
{
"_id" : ObjectId("5b7d9bbe1fbad410e10f32fa")
},
{
"_id" : ObjectId("5b7d9bc11fbad441494a663f")
},
{
"_id" : ObjectId("5b7d9bc41fbad4ff97273403")
},
{
"_id" : ObjectId("5b7d9bc71fbad4ff99527e83")
},
{
"_id" : ObjectId("5b7d9bca1fbad410de638bbd")
},
{
"_id" : ObjectId("5b7d9bcd1fbad441494a6640")
},
{
"_id" : ObjectId("5b7d9bd01fbad4ff97273404")
}
]
}
{
"_id" : ObjectId("5b7af9701fbad410e10f32f7"),
"preview" : {
"photo" : {
"id" : ObjectId("5b7b00351fbad441494a663d"),
"sort" : 0,
"primary" : false
}
},
"media" : [
{
"_id" : ObjectId("5b7affea1fbad441494a663b")
},
{
"_id" : ObjectId("5b7b002d1fbad441494a663c")
},
{
"_id" : ObjectId("5b7b00351fbad441494a663d")
},
{
"_id" : ObjectId("5b7d9baa1fbad410de638bbb")
},
{
"_id" : ObjectId("5b7d9bae1fbad410e10f32f9")
},
{
"_id" : ObjectId("5b7d9bb11fbad441494a663e")
},
{
"_id" : ObjectId("5b7d9bb41fbad4ff97273402")
},
{
"_id" : ObjectId("5b7d9bb71fbad4ff99527e82")
},
{
"_id" : ObjectId("5b7d9bbb1fbad410de638bbc")
},
{
"_id" : ObjectId("5b7d9bbe1fbad410e10f32fa")
},
{
"_id" : ObjectId("5b7d9bc11fbad441494a663f")
},
{
"_id" : ObjectId("5b7d9bc41fbad4ff97273403")
},
{
"_id" : ObjectId("5b7d9bc71fbad4ff99527e83")
},
{
"_id" : ObjectId("5b7d9bca1fbad410de638bbd")
},
{
"_id" : ObjectId("5b7d9bcd1fbad441494a6640")
},
{
"_id" : ObjectId("5b7d9bd01fbad4ff97273404")
}
]
}
and what you can se the last data have preview.photo.primary on it, and this field i want to return when i'm done with my aggregate query.
My final query look like this:
db.getCollection('travel_sights').aggregate([{
'$match': {
'preview.photo' : {
'$exists':true
},
'_id': {
'$in' : [ObjectId("5b7af9701fbad410e10f32f7")]
}
}
},{
'$unwind' : '$preview.photo'
}, {
'$lookup':{
'from' : 'media_data',
'localField' : '_id',
'foreignField':'bind',
'as':'media'
}
},{
'$unwind':'$media'
},{
'$project' : {
'preview' : 1,
'media': 1,
}
}, {
'$group': {
'_id':'$media._id',
'primary': {
'$first':'$preview'
}
}
}])
The problem here is when i want $preview return so i can find the primary about it, its allways only return the first where the value not exists, if i use $push the problem is i get every thing.
is there a way so i can pick the right primary value in my return? have trying $addFields to but whitout eny kind of lock.
Travel_sights data:
{
"_id" : ObjectId("5b7af9701fbad410e10f32f7"),
"city_id" : ObjectId("5b6d0cb6222d4c70b803eaeb"),
"activated" : true,
"deleted" : false,
"url" : "url is here",
"name" : "title of it here",
"updated_at" : ISODate("2018-08-22T17:22:27.000Z"),
"content" : "content here",
"preview" : {
"photo" : [
{
"id" : ObjectId("5b7affea1fbad441494a663b"),
"sort" : 0
},
{
"id" : ObjectId("5b7b002d1fbad441494a663c"),
"sort" : 0
},
{
"id" : ObjectId("5b7b00351fbad441494a663d"),
"sort" : 0,
"primary" : true
},
{
"id" : ObjectId("5b7d9baa1fbad410de638bbb"),
"sort" : 0
},
{
"id" : ObjectId("5b7d9bae1fbad410e10f32f9"),
"sort" : 0
},
{
"id" : ObjectId("5b7d9bb11fbad441494a663e"),
"sort" : 0
},
{
"id" : ObjectId("5b7d9bb41fbad4ff97273402"),
"sort" : 0,
"primary" : false
},
{
"id" : ObjectId("5b7d9bb71fbad4ff99527e82"),
"sort" : 0,
"primary" : false
},
{
"id" : ObjectId("5b7d9bbb1fbad410de638bbc"),
"sort" : 0
},
{
"id" : ObjectId("5b7d9bbe1fbad410e10f32fa"),
"sort" : 0
},
{
"id" : ObjectId("5b7d9bc11fbad441494a663f"),
"sort" : 0
},
{
"id" : ObjectId("5b7d9bc41fbad4ff97273403"),
"sort" : 0,
"primary" : false
},
{
"id" : ObjectId("5b7d9bc71fbad4ff99527e83"),
"sort" : 0,
"primary" : false
},
{
"id" : ObjectId("5b7d9bca1fbad410de638bbd"),
"sort" : 0,
"primary" : false
},
{
"id" : ObjectId("5b7d9bcd1fbad441494a6640"),
"sort" : 0,
"primary" : false
},
{
"id" : ObjectId("5b7d9bd01fbad4ff97273404"),
"sort" : 0
}
]
}
}
3 sample foto bind data here:
{
"_id" : ObjectId("5b7affea1fbad441494a663b"),
"file-name" : "55575110311__0F115282-B5A0-4654-AA44-B7DC2C682992.jpeg",
"options" : [
ObjectId("5b6fb855222d4c70b8041093")
],
"type" : "images",
"files" : [
{
"width" : 70,
"height" : 53
},
{
"width" : 400,
"height" : 300
},
{
"width" : 800,
"height" : 600
},
{
"width" : 1600,
"height" : 1200
}
],
"bind" : [
ObjectId("5b7af9701fbad410e10f32f7")
]
}
{
"_id" : ObjectId("5b7b002d1fbad441494a663c"),
"file-name" : "55575110748__E7B07EFD-9F7E-40D6-8B57-38F708E4C0C0.jpeg",
"options" : [
ObjectId("5b6fb855222d4c70b8041093")
],
"type" : "images",
"files" : [
{
"width" : 70,
"height" : 53
},
{
"width" : 400,
"height" : 300
},
{
"width" : 800,
"height" : 600
},
{
"width" : 1600,
"height" : 1200
}
],
"bind" : [
ObjectId("5b7af9701fbad410e10f32f7")
],
"description" : "this is secoudn demo!",
"title" : "demo 3"
}
{
"_id" : ObjectId("5b7b00351fbad441494a663d"),
"file-name" : "paris2.jpg",
"options" : [
ObjectId("5b6fb855222d4c70b8041093")
],
"type" : "images",
"files" : [
{
"width" : 70,
"height" : 53
},
{
"width" : 400,
"height" : 300
},
{
"width" : 800,
"height" : 600
},
{
"width" : 1600,
"height" : 1200
}
],
"bind" : [
ObjectId("5b7af9701fbad410e10f32f7")
],
"description" : "this is a demo1 :)",
"title" : "demo"
}

Answer 1

You can filter out the element from the array where the primary field exists using $filter aggregation and then easily $group with the media._id field and get the $first document value.
Finally your query will be
db.getCollection("travel_sights").aggregate([
{ "$match": {
"preview.photo" : { "$exists":true },
"_id": { "$in" : [ ObjectId("5b7af9701fbad410e10f32f7") ] }
}},
{ "$addFields": {
"preview.photo": {
"$arrayElemAt": [
{ "$filter": {
"input": "$preview.photo",
"as": "photo",
"cond": { "$ne": [ "$$photo.primary", undefined ] }
}}, 0
]
}
}},
{ "$lookup":{
"from" : "media_data",
"localField" : "_id",
"foreignField": "bind",
"as": "media"
}},
{ "$unwind":"$media" },
{ "$project" : { "preview" : 1, "media": 1, }},
{ "$group": {
"_id": "$media._id",
"primary": { "$first": "$preview" }
}}
])