Mongodb array concatenation - mongodb

Question

When querying mongodb, is it possible to process ("project") the result so as to perform array concatenation?
I actually have 2 different scenarios:
(1) Arrays from different fields:, e.g:
Given:
{companyName:'microsoft', managers:['ariel', 'bella'], employees:['charlie', 'don']}
{companyName:'oracle', managers:['elena', 'frank'], employees:['george', 'hugh']}
I'd like my query to return each company with its 'managers' and 'employees' concatenated:
{companyName:'microsoft', allPersonnel:['ariel', 'bella','charlie', 'don']}
{companyName:'oracle', allPersonnel:['elena', 'frank','george', 'hugh']}
(2) Nested arrays:, e.g.:
Given the following docs, where employees are separated into nested arrays (never mind why, it's a long story):
{companyName:'microsoft', personnel:[ ['ariel', 'bella'], ['charlie', 'don']}
{companyName:'oracle', personnel:[ ['elena', 'frank'], ['george', 'hugh']}
I'd like my query to return each company with a flattened 'personal' array:
{companyName:'microsoft', allPersonnel:['ariel', 'bella','charlie', 'don']}
{companyName:'oracle', allPersonnel:['elena', 'frank','george', 'hugh']}
I'd appreciate any ideas, using either 'find' or 'aggregate'
Thanks a lot :)

Answer 1

Of Course in Modern MongoDB releases we can simply use $concatArrays here:
db.collection.aggregate([
{ "$project": {
"companyNanme": 1,
"allPersonnel": { "$concatArrays": [ "$managers", "$employees" ] }
}}
])
Or for the second form with nested arrays, using $reduce in combination:
db.collection.aggregate([
{ "$project": {
"companyName": 1,
"allEmployees": {
"$reduce": {
"input": "$personnel",
"initialValue": [],
"in": { "$concatArrays": [ "$$value", "$$this" ] }
}
}
}}
])
There is the $setUnion operator available to the aggregation framework. The constraint here is that these are "sets" and all the members are actually "unique" as a "set" requires:
db.collection.aggregate([
{ "$project": {
"companyname": 1,
"allPersonnel": { "$setUnion": [ "$managers", "$employees" ] }
}}
])
So that is cool, as long as all are "unique" and you are in singular arrays.
In the alternate case you can always process with $unwind and $group. The personnel nested array is a simple double unwind
db.collection.aggregate([
{ "$unwind": "$personnel" },
{ "$unwind": "$personnel" },
{ "$group": {
"_id": "$_id",
"companyName": { "$first": "$companyName" },
"allPersonnel": { "$push": { "$personnel" } }
}}
])
Or the same thing as the first one for versions earlier than MongoDB 2.6 where the "set operators" did not exist:
db.collection.aggregate([
{ "$project": {
"type": { "$const": [ "M", "E" ] },
"companyName": 1,
"managers": 1,
"employees": 1
}},
{ "$unwind": "$type" },
{ "$unwind": "$managers" },
{ "$unwind": "$employees" },
{ "$group": {
"_id": "$_id",
"companyName": { "$first": "$companyName" },
"allPersonnel": {
"$addToSet": {
"$cond": [
{ "$eq": [ "$type", "M" ] },
"$managers",
"$employees"
]
}
}
}}
])