regroup after unwind of subdocument of subdocument - mongodb

Question

This is my Document.
{
"_id" : ObjectId("589b6132fafb5a09549b46cb"),
"name" : "foo",
"users" : [
{
"_id" : ObjectId("589b6132fafb5a09549b46cc"),
"name" : "Peter",
"emails" : [
{
"address" : "peter#email.com"
},
{
"address" : "test2#email.com"
}
]
},
{
"_id" : ObjectId("589b6132fafb5a09549b46cd"),
"name" : "Joe",
"emails" : []
}
]
}
I'm unwinding users and users.email
And when I try to regroup, I get a duplicate on user named Peter because it has 2 emails.
Query:
db.test.aggregate([
{ "$unwind": {
"path": "$users",
"preserveNullAndEmptyArrays": true
} },
{ "$unwind": {
"path": "$users.emails",
"preserveNullAndEmptyArrays": true
} },
{
"$group": {
"_id": "$_id",
"name": { "$first": "$name" },
"users": { "$addToSet": "$users"},
"allEmails": { "$push": "$users.emails.address" }
}
}
])
Result:
{
"_id" : ObjectId("589b6132fafb5a09549b46cb"),
"name" : "foo",
"users" : [
{
"_id" : ObjectId("589b6132fafb5a09549b46cd"),
"name" : "Joe"
},
{
"_id" : ObjectId("589b6132fafb5a09549b46cc"),
"name" : "Peter",
"emails" : {
"address" : "test2#email.com"
}
},
{
"_id" : ObjectId("589b6132fafb5a09549b46cc"),
"name" : "Peter",
"emails" : {
"address" : "peter#email.com"
}
}
],
"allEmails" : [
"peter#email.com",
"test2#email.com"
]
}
I need the users object to be exact the same before the unwind with allEmails on the document parent as shown in the following example.
{
"_id" : ObjectId("589b6132fafb5a09549b46cb"),
"name" : "foo",
"users" : [
{
"_id" : ObjectId("589b6132fafb5a09549b46cc"),
"name" : "Peter",
"emails" : [
{ "address" : "test2#email.com" },
{ "address" : "peter#email.com" }
]
},
{
"_id" : ObjectId("589b6132fafb5a09549b46cd"),
"name" : "Joe",
"emails" : []
}
],
"allEmails" : [
"peter#email.com",
"test2#email.com"
]
}

Answer 1

Running the following aggregate pipeline should give you the desired result:
db.test.aggregate([
{
"$addFields": {
"allEmails": {
"$reduce": {
"input": {
"$map": {
"input": "$users",
"as": "user",
"in": "$$user.emails"
}
},
"initialValue": [],
"in": { "$concatArrays": ["$$value", "$$this.address"] }
}
}
}
}
])
The above pipeline works by initially creating a two dimensional array of emails addresses objects using $map. To show an example result produced by apply the expression
{
"$map": {
"input": "$users",
"as": "user",
"in": "$$user.emails"
}
}
run a test pipeline with just a single field that holds the results:
db.test.aggregate([
{
"$project": {
"twoDarray": {
"$map": {
"input": "$users",
"as": "user",
"in": "$$user.emails"
}
}
}
}
}
])
which will produce the 2D array
{
"_id" : ObjectId("589b6132fafb5a09549b46cb"),
"twoDarray" : [
[
{ "address" : "peter#email.com" },
{ "address" : "test2#email.com" }
],
[]
]
}
Now, denormalise this 2-D array
[
[
{ "address" : "peter#email.com" },
{ "address" : "test2#email.com" }
],
[]
]
by using the $reduce operator which applies an expression to each element in an array and combines them into a single value. With the help of the $concatArrays operator, you can concatenate each element within the $reduce expression to form the final desired array
[
"peter#email.com",
"test2#email.com"
]