How to Join two collections in MongoDB

To join MongoDB collections, you can use the MongoDB Aggregation Pipeline, which provides a set of operators to perform operations such as grouping, filtering, and transforming data. The $lookup operator is used to perform a left outer join between two collections.

Here is an example of how to join two collections in MongoDB using the $lookup operator:

db.orders.aggregate([
  {
    $lookup: {
      from: "products",
      localField: "product_id",
      foreignField: "_id",
      as: "product"
    }
  }
])

In this example, we have two collections, “orders” and “products”. The localField parameter specifies the field from the “orders” collection to join on, and the foreignField parameter specifies the field from the “products” collection to join on. The as parameter specifies the name of the new field that will hold the joined data.

This query will return all documents from the “orders” collection, with a new field called “product” that contains the joined data from the “products” collection based on the _id field.

Note that the syntax and parameters may vary depending on the version of MongoDB and the specific use case. For more information on the $lookup and other MongoDB Aggregation Pipeline operators, you can refer to the MongoDB documentation.

Let’s take a look at real time example of how to join two collection in mongoDB.

Let’s say you have two collections in your MongoDB database: “users” and “posts”. The “users” collection has documents that represent users, and the “posts” collection has documents that represent posts made by those users. Each post document has a reference to the user who made the post using the user’s _id field.

Now, if you want to retrieve all posts with the corresponding user information, you can use the $lookup operator to perform a left outer join between the “users” and “posts” collections. Here’s an example query:

db.posts.aggregate([
  {
    $lookup: {
      from: "users",
      localField: "user_id",
      foreignField: "_id",
      as: "user"
    }
  }
])

In this query, the from parameter specifies the name of the “users” collection, the localField parameter specifies the field from the “posts” collection to join on (in this case, “user_id”), and the foreignField parameter specifies the field from the “users” collection to join on (in this case, “_id”). The as parameter specifies the name of the new field that will hold the joined data (in this case, “user”).

This query will return all documents from the “posts” collection, with a new field called “user” that contains the joined data from the “users” collection based on the user_id field.

This is a real-time example of how you can use the $lookup operator in MongoDB to join two collections and retrieve related data in a single query.

Hope this article helped you find how to join two collections in mongodb.

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