How to group over Array elements using Mongoose Aggregation FrameWork - node.js

Question

I have following Mongoose schemas :
EmployeeSchema :
var EmployeeSchema = new Schema({
name : String,
employeeDetailsId: {
type: Schema.Types.ObjectId,
ref: 'employeedetails'
}
});
EmployeeDetailSchema :
var EmployeeDetailSchema = new Schema({
employeeId: {
type: Schema.Types.ObjectId,
ref: 'employee'
},
statusId: {
type: Schema.Types.ObjectId,
ref: 'status'
},
primarySkills: [
{
type: Schema.Types.ObjectId,
ref: 'skills'
}]
});
SkillsSchema :
var SkillsSchema = new Schema({
name: {
type: String,
required: true
}
});
Following is the data that's got saved in EmployeeDetails collection :
/* 1 */
{
"_id" : ObjectId("583fbbfe78854dd424f0523f"),
"employeeId" : ObjectId("583f114e1cff44b7ab414dc1"),
"statusId" : ObjectId("583ee05a1d5161941632091a"),
"secondarySkills" : [],
"primarySkills" : [],
"__v" : 0
}
/* 2 */
{
"_id" : ObjectId("583ff108cfa71d942269b09b"),
"employeeId" : ObjectId("583f114e1cff44b7ab414dc4"),
"statusId" : ObjectId("583ee05a1d5161941632091a"),
"secondarySkills" : [],
"primarySkills" : [],
"__v" : 0
}
/* 3 */
{
"_id" : ObjectId("5848c40599fa37d40a7e7392"),
"employeeId" : ObjectId("583f114e1cff44b7ab414dc8"),
"secondarySkills" : [
ObjectId("5838373072d7bab017488ba2")
],
"primarySkills" : [
ObjectId("5848c3c299fa37d40a7e7390"),
ObjectId("5848c3d599fa37d40a7e7391")
],
"__v" : 0
}
/* 4 */
{
"_id" : ObjectId("5848c41699fa37d40a7e7393"),
"employeeId" : ObjectId("583f114e1cff44b7ab414dc6"),
"secondarySkills" : [],
"primarySkills" : [
ObjectId("5838373072d7bab017488ba2"),
ObjectId("5848c3c299fa37d40a7e7390"),
ObjectId("5848c3d599fa37d40a7e7391")
],
"__v" : 0
}
UseCase :
When i want to group EmployeeDetails collection based on Status ID, i used following aggregation in Mongoose :
EmployeeDetailsModel.aggregate([
{
$group: {_id: "$statusId", count: {$sum: 1}}
}
]).exec(...);
In similar way, i want to group based on primarySkills or secondarySkills where both of them are array of Skill ObjectID's.
I tried few approaches but no luck. Need some help.

Answer 1

So if you are trying to get a result that shows a list of employees who has a certain skill, $unwind might help.
db.emp.aggregate([{$unwind:"$primarySkills"},{$group:{"_id":"$primarySkills", "employees":{$push:"$employeeId"}}}])
And here's the result:
{ "_id" : ObjectId("5848c3d599fa37d40a7e7391"), "employees" : [ ObjectId("583f114e1cff44b7ab414dc6"), ObjectId("583f114e1cff44b7ab414dc8") ] }
{ "_id" : ObjectId("5848c3c299fa37d40a7e7390"), "employees" : [ ObjectId("583f114e1cff44b7ab414dc6"), ObjectId("583f114e1cff44b7ab414dc8") ] }
{ "_id" : ObjectId("5838373072d7bab017488ba2"), "employees" : [ ObjectId("583f114e1cff44b7ab414dc6") ] }
The $unwind doc.