Differences between $project, $filter and $match in MongoDB

Question

I'm new to MongoDB and to NoSQL servers in general. I'm testing some stuffs (insert in local DB, easy queries, etc..) on my computer with Studio 3T software (IDE for MongoDB) and I got into these 3 operators while testing aggregate method: $project, $filter and $match. The problem is I don't get the differences between these operators, the only thing I think i might understood is that $project is used to "choose" which fields (or whole array) to show on screen in response from the query, but what about $match and $filter? they seem to do the same thing. I read MongoDB documentation but it doesn't explain the operators sufficiently detailed (in my opinion of course).

Thanks in advance

Answer 1

$project and $match are Aggregation Pipeline Stages and $filter is an Aggregation Pipeline Operator.

Difference between Pipeline stage and Pipeline operator is a stage can used separately, whereas a Pipeline operator can be used only inside a Pipeline stage.

$project is used to project the elements to next stage

$match is used to filter the collection to give only the matching documents for the input query/criteria and it is advisable to use it at the start of Aggregation Pipeline before using other operators. Since it filters the collection to a reduced number of documents and only reduced set of documents is passed to the next stage of Aggregation Pipeline.

$filter is used in arrays, it selects a subset of an array to return based on the specified condition. Returns an array with only those elements that match the condition. The returned elements are in the original order.