MongoEngine - Indexes

An indexed collection results in faster processing of queries. By default, every collection is automatically indexed on _id field. In addition, you can create index on one or more fields.

Using Compass, we can build index very easily. Cpck on CREATE INDEX button on Indexes tab as shown in figure below−

A dialog box appears as shown. Choose name of index, field on which to index, order of index (ascending or descending) and other options.

While using MongoEngine, indexes are created by specifying ‘indexes’ key in meta dictionary of definition of Document class.

Value of indexes property is a pst of fields. In the following example, we ask documents in student collection be indexed according to name field.

from mongoengine import *
con=connect( mydata )
class student(Document):
   name=StringField(required=True)
   course=StringField()
   meta = { indexes :[ name ]}
   
s1=student()
s1.name= Avinash 
s1.course= DataScience 
s1.save()
s2=student()
s2.name= Anita 
s2.course= WebDesign 
s2.save()

By default, indexing order is ascending. Order may be specified by prepending ‘+’ for ascending or ‘-‘ for descending order.

To create compound index, use a tuple of field names, optionally having + or – symbol attached to indicate sort order.

In the following example, student document class contains definition of compound index on name and course (note - symbol prefixed to course field which means index is built namewise ascending and coursewise descending order.

from mongoengine import *
con=connect( mydata )

class student(Document):
   name=StringField(required=True)
   course=StringField()
   meta = { indexes :[( name , -course )]}

s1=student()
s1.name= Avinash 
s1.course= DataScience 
s1.save()
s2=student()
s2.name= Anita 
s2.course= WebDesign 
s2.save()

MongoDB Compass will show indexes as below −

Value of ‘indexes’ may be a dictionary of various options as below −

Following example creates index on name field that expires after 3600 seconds.

from mongoengine import *
con=connect( mydata )

class student(Document):
   name=StringField(required=True)
   course=StringField()
   meta = { indexes :[{
             fields : [ name ],
             expireAfterSeconds : 3600
           }
    ]
}

To specify text index, prefix field name with ‘$’ sign and for hashed index, use ‘#’ as prefix.

Indexes so specified are created automatically as documents are added in the collection. To disable automatic creation, set ‘auto_create_index’ to False in meta attribute.

We have pst_indexes() method with Document class that displays pst of available indexes.

print (student.pst_indexes())

[[( name , 1)], [( _id , 1)]]

To create index on a field not in the meta dictionary, use create_index() method. The following code will create index on course field −

class student(Document):
name=StringField(required=True)
course=StringField()
meta = { indexes :[{
           fields : [ name ],
           expireAfterSeconds : 3600
        }
]}
student.create_index([ course ])