MongoDB aggregate lookup with nested array - mongodb

Question

I have a complicated structure I am trying to "join".
The best way to describe it is that I have "Favorite Teams" stored with a user, as an array of name/IDs - however they are stored in a nested object. I want to return the users Favorite Teams Players WITH the team.
Here are the data models
PLAYERS
{
_id:
team_id:
name:
position:
}
TEAMS
{
_id:
name:
}
USER
{
_id:
name:
favs: {
mascots: [{
_id:
name:
}],
teams: [{
_id:
name:
}],
}
}
I have an array of Team IDs from the user.favs.teams - and what I want back is the players with their team name.
This is the current aggregation I am using - it is returning the players but not the teams...I am pretty sure I need to unwind, or similar.
players.aggregate([
{
$match: {
team_id: {
$in: [--array of team ID's--]
}
}
},
{
$lookup: {
from: 'teams',
localField: 'team_id',
foreignField: '_id',
as: 'players_team'
}
},
{
$project: {
_id: 1,
name: 1,
position: 1,
'players_team[0].name': 1
}
}
])
What I am getting back...
_id: 5c1b37b6fd15241940b11111
name:"Bob"
position:"Test"
team_id:5c1b37b6fd15241940b441dd
player_team:[
_id:5c1b37b6fd15241940b441dd
name:"Team A"
...other fields...
]
What I WANT to get back...
_id: 5c1b37b6fd15241940b11111
name:"Bob"
position:"Test"
team_id:5c1b37b6fd15241940b441dd
player_team: "Team A"

Answer 1

Use Below $lookup (Aggregation)
db.players.aggregate([
{
$lookup: {
from: "teams",
let: { teamId: "$team_id" },
pipeline: [
{
$match: { $expr: { $eq: [ "$_id", "$$teamId" ] } }
},
{
$project: { _id: 0 }
}
],
as: "players_team"
}
},
{
"$replaceRoot": {
"newRoot": {
"$mergeObjects": [
{
"_id": "$_id",
"name": "$name",
"position": "$position",
"team_id": "$team_id"
},
{
player_team: { $arrayElemAt: [ "$players_team.name", 0 ] }
}
]
}
}
}
])
Sorry If your MongoDB version is less then 3.6. Because of new changes in MongoDB 3.6.