$first (aggregation)
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Definition
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$first
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Returns the value that results from applying an expression to the first document in a group of documents that share the same group by key. Only meaningful when documents are in a defined order.
$first
is only available in the$group
stage.$first
has the following syntax:For more information on expressions, see Expressions.
Behavior
When using $first
in a $group
stage, the $group
stage should follow a $sort
stage to have the input documents in a defined order.
Example
Consider a sales
collection with the following documents:
Grouping the documents by the item
field, the following operation uses the $first
accumulator to compute the first sales date for each item:
The operation returns the following results: