Custom expression operators include $accumulator and $function. There are many categories of expression operators: arithmetic, Boolean, comparison, conditional, data size, date, literal, object, set, string, text, trigonometry, type, etc. By becoming familiar with these, developers can simplify their application code by doing most of the processing in MongoDB. Source: Adapted from MongoDB Docs 2021h.Īn expression operator has a name and takes either an array of arguments or a single argument. What are expression operators in MongoDB aggregation?Īveraging as accumulator and non-accumulator.$unwind: Given an array field, outputs one document per array element.$sort: Reorders document stream by a specified sort key.Operator $unset is an alias for removing fields. $project: Reshapes documents by adding or removing fields.$merge: Writes the output to a collection.$match: Filters documents based on standard MongoDB queries. $lookup: Does a left outer join to another collection in the same database.$limit: Passes only the first n documents to the next stage.If specified, applies an accumulator expression for each group. $group: Takes an identifier expression for grouping input documents.$count: Count number of documents at this stage.$addFields: Add new fields to documents.Except for $out, $merge and $geoNear, other stages can appear multiple times in a pipeline. Could you describe some aggregation pipeline operators or stages?Īn aggregation pipeline is constructed with the aggregate() method of the Collection class and takes an array of stages: db.collection.aggregate( ). While handy, these lack the flexibility and capability of the aggregation pipeline.
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