MongoDB Aggregate Pipeline Nested

Main Page >> MongoDB >>MongoDB Workbook >> Aggregation Pipeline and nested data

Aggregation Pipeline

So far find() either returns all the elements of an array, if one element matches the search criteria, or $elematch returns the first one found only. The latter is fine if there is only one to be found, but not so good if several items in the array should match the search criteria. The aggregation pipeline can help with this.

$filter

In the aggregation pipeline, array has various operators and the one we are interested in is $filter

This returns a subset of the array with only the elements that match the filter condition.

$filter has the following syntax:

{ $filter: {
	input: <array>,    /* expression for the array */
	as: <string>, 	   /* variable name for the element */   
	cond: <expression> /* filter condition */
} }

$filter is one of the stages of the pipeline and can not be used by itself. It is used with an aggregation framework operator, such as $project.

We can use the pipeline to gather elements of our employees array to get the employees matching the query criteria only, rather than one, or everyone. This example has only one stage:

db.deptCollection.aggregate([ {
     $project: {
        empSet: {
           $filter: {
              input: "$employees",
              as: "employee",
              cond: { $gte: [ "$$employee.sal", 2000 ] }
           }
        }
     }
  }
]).pretty()

Now the system should only retrieve the employees with salary > 2000.

Count

The power of the aggregation pipeline is to do processing on the data.

Lets count how many employees each department has:

db.deptCollection.aggregate({ 
 "$project": {
    "deptno": 1,
    "Count": { "$size": { "$ifNull": [ "$employees", [] ] }
     }
   }})

The $ifNull operator is needed, since department 40 has no employees - you will get an error message if left out!

Next Step

Updating a collection with nested data