Question 118411 in Mongodb Question posted in
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Mongodb – Pymongo how to set read_preference

How do we enable PyMongo to read from the nearest? This field db.read_preference is read-only now.

from pymongo import ReplicaSetConnection
from pymongo import ReadPreference

db = ReplicaSetConnection('localhost:27017', replicaSet='rs1')['my_db']
db.read_preference = ReadPreference.NEAREST
db.tag_sets = [{'secondaries': 1}]
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3

Answers


  1. 0

    You need to add it to the connection string, like so:

    db = ReplicaSetConnection('localhost:27017?readPreference=nearest', replicaSet='rs1')['my_db']

    Or

    db = MongoClient("myhost:27017", read_preference=ReadPreference.NEAREST, replicaSet='rs1')['my_db']

    For whatever reason the given syntax you’re using fails, it seems it’s the syntax supported for older Pymongo versions.

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  2. 0

    Option 1(per connection):

    >>> from pymongo import MongoClient, ReadPreference
>>> db = MongoClient("myhost:27017", read_preference=ReadPreference.NEAREST).test
>>> db.collection.find_one({test:1})

    Option 2(per query):

    >>> db.collection.find_one({test:1},read_preference=ReadPreference.NEAREST)
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  3. 0

    PyMongo ReadPreference can also be applied at database, collection or a query level (in addition to within Connection String URI and with the MongoClient).

    from pymongo import ReadPreference

# Specifying read preference at database level
db1 = client.testdb1
db2 = client.get_database('testdb2', read_preference=ReadPreference.NEAREST)

# Specifying read preference at collection level
coll2 = db1.coll1.with_options(read_preference=ReadPreference.NEAREST)

# Specifying read preference for a specific query
result = db1.coll1.with_options(read_preference=ReadPreference.NEAREST).find()

    Note the read preference set at the lower level (e.g., collection) will override the one set at the previous level (e.g., database or mongo client).

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