If you're working with MongoDB and need to combine data from multiple collections, left outer join can be a useful tool. Here's how to use it in MongoDB:
What is Left Outer Join in MongoDB?
Left outer join is a type of database join that combines data from two collections based on a matching condition, but also includes non-matching data from the left collection. This means that all the data from the left collection is returned, even if there is no matching data in the right collection.
How to Perform a Left Outer Join in MongoDB
To perform a left outer join in MongoDB, you can use the $lookup aggregation pipeline stage. Here's an example:
const client= await clientPromise;
const db=client.db("blog");
const records = await db.collection("products").aggregate([
{
$lookup: {
from: "categories",
localField: "category_id",
foreignField: "_id",
as: "category"
}
},
{
$unwind:"$category"
}
])
.toArray();
res.json(records);
In this example, we're joining data from the "products" and "categories" collections based on the "category_id" and "_id" fields, respectively.
Example of Left Outer Join in MongoDB
Let's say we have two collections, "products" and "categories", with the following data:
// products collection
{ "_id" : 1, "name" : "Product A", "category_id" : 1 }
{ "_id" : 2, "name" : "Product B", "category_id" : 2 }
{ "_id" : 3, "name" : "Product C", "category_id" : 1 }
// categories collection
{ "_id" : 1, "name" : "Category A" }
{ "_id" : 2, "name" : "Category B" }
If we want to join the two collections based on the "category_id" and "_id" fields, we can use the following MongoDB query:
db.products.aggregate([
{
$lookup: {
from: "categories",
localField: "category_id",
foreignField: "_id",
as: "category"
}
},
{
$unwind:"$category"
}
])
This will return the following result:
{ "_id" : 1, "name" : "Product A", "category_id" : 1, "category" : { "_id" : 1, "name" : "Category A" } }
{ "_id" : 2, "name" : "Product B", "category_id" : 2, "category" : { "_id" : 2, "name" : "Category B" } }
{ "_id" : 3, "name" : "Product C", "category_id" : 1, "category" : { "_id" : 1, "name" : "Category A" } }