MongoDB 3.2 now allows one to combine data from multiple collections into one through the $lookup aggregation stage. As a practical example, lets say that you have data about books split into two different collections.
First collection, called books
, having the following data:
{
"isbn": "978-3-16-148410-0",
"title": "Some cool book",
"author": "John Doe"
}
{
"isbn": "978-3-16-148999-9",
"title": "Another awesome book",
"author": "Jane Roe"
}
And the second collection, called books_selling_data
, having the following data:
{
"_id": ObjectId("56e31bcf76cdf52e541d9d26"),
"isbn": "978-3-16-148410-0",
"copies_sold": 12500
}
{
"_id": ObjectId("56e31ce076cdf52e541d9d28"),
"isbn": "978-3-16-148999-9",
"copies_sold": 720050
}
{
"_id": ObjectId("56e31ce076cdf52e541d9d29"),
"isbn": "978-3-16-148999-9",
"copies_sold": 1000
}
To merge both collections is just a matter of using $lookup in the following way:
db.books.aggregate([{
$lookup: {
from: "books_selling_data",
localField: "isbn",
foreignField: "isbn",
as: "copies_sold"
}
}])
After this aggregation, the books
collection will look like the following:
{
"isbn": "978-3-16-148410-0",
"title": "Some cool book",
"author": "John Doe",
"copies_sold": [
{
"_id": ObjectId("56e31bcf76cdf52e541d9d26"),
"isbn": "978-3-16-148410-0",
"copies_sold": 12500
}
]
}
{
"isbn": "978-3-16-148999-9",
"title": "Another awesome book",
"author": "Jane Roe",
"copies_sold": [
{
"_id": ObjectId("56e31ce076cdf52e541d9d28"),
"isbn": "978-3-16-148999-9",
"copies_sold": 720050
},
{
"_id": ObjectId("56e31ce076cdf52e541d9d28"),
"isbn": "978-3-16-148999-9",
"copies_sold": 1000
}
]
}
It is important to note a few things:
- The “from” collection, in this case
books_selling_data
, cannot be sharded. - The “as” field will be an array, as the example above.
- Both “localField” and “foreignField” options on the $lookup stage will be treated as null for matching purposes if they don’t exist in their respective collections (the $lookup docs has a perfect example about that).
So, as a conclusion, if you want to consolidate both collections, having, in this case, a flat copies_sold field with the total copies sold, you will have to work a little bit more, probably using an intermediary collection that will, then, be $out to the final collection.