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$or (aggregation)

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Definition

$or

Evaluates one or more expressions and returns true if any of the expressions are true. Otherwise, $or returns false.

$or has the following syntax:

{ $or: [ <expression1>, <expression2>, ... ] }

For more information on expressions, see Expressions.

Behavior

$or uses short-circuit logic: the operation stops evaluation after encountering the first true expression.

In addition to the false boolean value, $or evaluates as false the following: null, 0, and undefined values. The $or evaluates all other values as true, including non-zero numeric values and arrays.

Example   Result
{ $or: [ true, false ] }   true
{ $or: [ [ false ], false ] }   true
{ $or: [ null, 0, undefined ] }   false
{ $or: [ ] }   false

Example

Consider an inventory collection with the following documents:

{ "_id" : 1, "item" : "abc1", description: "product 1", qty: 300 }
{ "_id" : 2, "item" : "abc2", description: "product 2", qty: 200 }
{ "_id" : 3, "item" : "xyz1", description: "product 3", qty: 250 }
{ "_id" : 4, "item" : "VWZ1", description: "product 4", qty: 300 }
{ "_id" : 5, "item" : "VWZ2", description: "product 5", qty: 180 }

The following operation uses the $or operator to determine if qty is greater than 250 or less than 200:

db.inventory.aggregate(
   [
     {
       $project:
          {
            item: 1,
            result: { $or: [ { $gt: [ "$qty", 250 ] }, { $lt: [ "$qty", 200 ] } ] }
          }
     }
   ]
)

The operation returns the following results:

{ "_id" : 1, "item" : "abc1", "result" : true }
{ "_id" : 2, "item" : "abc2", "result" : false }
{ "_id" : 3, "item" : "xyz1", "result" : false }
{ "_id" : 4, "item" : "VWZ1", "result" : true }
{ "_id" : 5, "item" : "VWZ2", "result" : true }