Read: MongoDB sort by date MongoDB join two collections using python This will make a left outer join with another collection and assist to filters the documents from joined documents.īut there is no concept of joining the two collections from different databases in MongoDB so we can not join them. In MongoDB, when we join two collections in the same database then we do with the help of $lookup aggregation. Read: How to check if MongoDB is installed + MongoDB Version MongoDB join two collections from different database Now, if you do more practice with MongoDB compass then you will more understand the aggregation operations and be more familiar with the compass environment. This is the simplest way of applying the $lookup aggregation using MongoDB compass. After applying aggregation, you can see the output right side of the input.In from field write the collection name which you want to join(movies)and in localField and foreignField write the field of local collection(genrelist) and field of “form collection”(type) respectively and in as write output array field(movie). In this function, you have to provide some fields of the collection. In this example, we create two different collections and after that, we join the collection by id.Īpply $lookup aggregation using MongoDB compassĪfter selecting aggregation at the bottom, there is a “pipeline stage” where a dropdown list and where you have to select the operation which you want to perform and here we select the $lookup function. Here, I’ll solve an example that will help you to understand how to join collection by their id in MongoDB. As you already know that for joining the collection we use the $lookup aggregation. In this topic, you will learn how you can join two collections by id. Read: MongoDB group by count MongoDB join two collections by id So when you have an array of fields in documents and you want to merge the collections then this way you can easily join the collections. Here we apply $lookup aggregation on member collection using the field enrollmentlist of an array and merge the two collections with foreignField name and that displays the below output. Now, we apply aggregation operation and join documents of the members’ collection with classes collection. Here, these are some documents we inserted into the members’ collection. This is the simple syntax or query that is used to join two collections. In this, you have to specify which collection you want to join with the current collection and select the field that matches in both the collection. In MongoDB, we can combine data of multiple collections into one through the $lookup aggregation stage. Read: MongoDB group by multiple fields MongoDB join two collections query If documents come from the “joined” collection, the $lookup (aggregation) function will return the documents in the form of a sub-array of the target original collection. The $lookup(aggregation) is a stage that makes a left outer join with another collection and assists to filters the documents from joined documents. In MongoDB, the $lookup (aggregation) function allows us to join the documents on collections that reside in the same database. What is used for joining the collection in MongoDB?
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