Lookup in mongodb node js
my grades model is: Show
people model is:
my aggregate query is:
** but myCustomResut is empty result like myCustomResut []: ** what is wrong with this code?
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$lookup Changed in version 5.1. Performs a left outer join to a collection in the same database to filter in documents from the "joined" collection for processing. The
Starting in MongoDB 5.1, To combine elements from two different collections, use the The To perform an equality match between a field from the input documents with a field from the documents of the "joined" collection, the
The
The operation would correspond to the following pseudo-SQL statement:
See these examples:
MongoDB 3.6 adds support for:
In MongoDB, a correlated subquery is a pipeline in a NoteStarting in MongoDB 5.0, for an uncorrelated subquery in a MongoDB correlated subqueries are comparable to SQL correlated subqueries, where the inner query references outer query values. An SQL uncorrelated subquery does not reference outer query values. MongoDB 5.0 also supports concise correlated subqueries. To perform correlated and uncorrelated subqueries with two collections, and perform other join conditions besides a single equality match, use this
The
pipeline Specifies the The The NoteTo reference variables in pipeline stages, use the The
let variables can be accessed by the stages in the pipeline, including additional
as Specifies the name of the new array field to add to the joined documents. The new array field contains the matching documents from the joined collection. If the specified name already exists in the joined document, the existing field is overwritten. What is the use of lookup in MongoDB?$lookup performs an equality match on the localField to the foreignField from the documents of the from collection. If an input document does not contain the localField , the $lookup treats the field as having a value of null for matching purposes. Specifies the field from the documents in the from collection.
How fetch data from two tables in MongoDB?MongoDB is not a relational database, but you can perform a left outer join by using the $lookup stage. The $lookup stage lets you specify which collection you want to join with the current collection, and which fields that should match.
What is $project in MongoDB?Definition. $project. Passes along the documents with the requested fields to the next stage in the pipeline. The specified fields can be existing fields from the input documents or newly computed fields.
What is $match in MongoDB?$match takes a document that specifies the query conditions. The query syntax is identical to the read operation query syntax; i.e. $match does not accept raw aggregation expressions. Instead, use a $expr query expression to include aggregation expression in $match .
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