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Practical MongoDB Aggregations
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There are many types of stages in the aggregation framework that don't allow expressions to be embedded. Here are some examples of some of the most popular of these stages:
$
match
$
limit
$
skip
$
sort
$
count
$
lookup
$
out
Some of these stages may be a surprise to you if you've never really thought about it before. You might consider $match
to be the most surprising item in this list. The content of a $match
stage is just a set of query conditions with the same syntax as MongoDB Query Language rather than an aggregation expression. There is a good reason for this. The aggregation engine reuses the MongoDB Query Language query engine to perform a regular query against the collection, enabling the query engine to use all its usual optimizations. The query conditions are taken as-is from the $match
stage at the top of the pipeline. Therefore, the $match
filter must use the same syntax as MongoDB Query Language...