Python MongoDB - create_index Query

MongoDB is an open-source document-oriented database. MongoDB stores data in the form of key-value pairs and is a NoSQL database program. The term NoSQL means non-relational.

Indexing 

Indexing helps in querying the documents efficiently. It stores the value of a specific field or set of fields which are ordered by the value of the field as specified in the index. 

PyMongo contains a function create_index() to explicitly create index. By default, _id is the only index present in the collection. This function can accept either a key or a list of (key, direction) pairs.

Syntax: 

create_index(keys, session=None, **kwargs)

Let's look at some examples.

Example 1:

Sample Database:

from pymongo import MongoClient # creation of MongoClient  client = MongoClient() # Connect with the portnumber and host  client = MongoClient("mongodb://localhost:27017/") # Access database  mydatabase = client['GFG'] # Access collection of the database  mycollection = mydatabase['College'] # Before Creating index index_list = sorted(list(mycollection.index_information())) print("Before Creating index") print(index_list) # Creating index mycollection.create_index("student_id", unique = True) # After Creating index index_list = sorted(list(mycollection.index_information())) print("\nAfter Creating index") print(index_list) 

Output:

Before Creating index ['_id_'] After Creating index ['_id_', 'student_id_1']

1. Here, we create an index named student_id using create_index() method. This results in two indexes in the documents _id and student_id.
2. Using index_information() method, we get all the indexes in the collection,

Example 2:

from pymongo import MongoClient # creation of MongoClient  client = MongoClient() # Connect with the portnumber and host  client = MongoClient("mongodb://localhost:27017/") # Access database  mydatabase = client['GFG'] # Access collection of the database  mycollection = mydatabase['College'] record = {'_id': 4, "student_id": 873, "name": "John", "section": "A"} mycollection.insert_one(record) 

Output:

DuplicateKeyError                         Traceback (most recent call last) <ipython-input-62-264f0e13db93> in <module>     16 record = {'_id': 4, "student_id": 873, "name": "John", "section": "A"}     17 ---> 18 mycollection.insert_one(record)

DuplicateKeyError: E11000 duplicate key error collection: GFG.College index: student_id_1 dup key: { : 873 }

It raises the DuplicateKeyError as there is already a document that exists with the student_id 873 and we are trying to insert another document with the same student_id. This error occurs because we created an index on the field student_id and marked it as unique.