mongoose.aggregate([
    { "$match": { "weekNumber": weekNumber } },
    { "$unwind": "$locations" },
    { "$addFields": { "shifts": "$locations.shifts"}},
    { "$unwind": "$shifts" },
    { "$lookup": {
            "let": { "userObjId": { "$toObjectId": "$shifts.shiftTypeId" } },
            "from": "shiftTypes",
            "pipeline": [
                { "$match" : { "$expr": { "$eq" : [ "$_id", "$$userObjId"] } } }
            ],
            "as": "shiftType"
        }
    },
    { "$addFields": {
            "shifts.name": "$shiftType.name",
            "shifts.color": "$shiftType.color"
        }
    },
    { "$project": {
            "shiftType": 0,
            "locations.shifts": 0
        }
    }
])

[
  {
    _id: ObjectId(),
    "weekNumber": 30,
    "locations": [
      {
        "location": "locationName1",
        "shifts": [
          {
            "shiftTypeId": "shiftType001",
          },
          {
            "shiftTypeId": "shiftType002",
          },
          {
            "shiftTypeId": "shiftType001",
          }
        ]
      },
      {
        "location": "locationName2",
        "shifts": [
          {
            "shiftTypeId": "shiftType001",
          },
          {
            "shiftTypeId": "shiftType002",
          },
          {
            "shiftTypeId": "shiftType001",
          }
        ]
      },
      {
        "location": "locationName3",
        "shifts": [
          {
            "shiftTypeId": "shiftType001",
          },
          {
            "shiftTypeId": "shiftType002",
          },
          {
            "shiftTypeId": "shiftType001",
          }
        ]
      }
    ]
  }
]

[
  {
    _id: ObjectId("shiftType001"),
    "name": "shift001",
    "color": "red"
  },
  {
    _id: ObjectId("shiftType002"),
    "name": "shift002",
    "color": "blue"
  }
]

[
  {
    _id: ObjectId(),
    "weekNumber": 30,
    "locations": [
      {
        "location": "locationName1",
        "shifts": [
          {
            "shiftTypeId": "shiftType001",
            "name": "shift001",
            "color": "red"
          },
          {
            "shiftTypeId": "shiftType002",
            "name": "shift002",
            "color": "blue"
          },
          {
            "shiftTypeId": "shiftType001",
            "name": "shift001",
            "color": "red"
          }
        ]
      },
      {
        "location": "locationName2",
        "shifts": [
          {
            "shiftTypeId": "shiftType002",
            "name": "shift002",
            "color": "blue"
          },
          {
            "shiftTypeId": "shiftType002",
            "name": "shift002",
            "color": "blue"
          },
          {
            "shiftTypeId": "shiftType001",
            "name": "shift001",
            "color": "red"
          }
        ]
      },
      {
        "location": "locationName3",
        "shifts": [
          {
            "shiftTypeId": "shiftType001",
            "name": "shift001",
            "color": "red"
          },
          {
            "shiftTypeId": "shiftType001",
            "name": "shift001",
            "color": "red"
          },
          {
            "shiftTypeId": "shiftType001",
            "name": "shift001",
            "color": "red"
          }
        ]
      }
    ]
  }
]