In order to connect to MongoDB replica set in Python you can give all server node addersses to MongoClient. Addresses passed to MongoClient() are called the seeds. As long as at least one of the seeds is online, MongoClient discovers all the members in the replica set, and determines which is the current primary and which are secondaries or arbiters.


Sample usages:

>>> MongoClient('localhost', replicaset='foo')
     MongoClient(host=['localhost:27017'], replicaset='foo', ...)
 >>> MongoClient('localhost:27018', replicaset='foo')
     MongoClient(['localhost:27018'], replicaset='foo', ...)
 >>> MongoClient('localhost', 27019, replicaset='foo')
     MongoClient(['localhost:27019'], replicaset='foo', ...)
 >>> MongoClient('mongodb://localhost:27017,localhost:27018/?replicaSet=foo')
     MongoClient(['localhost:27017', 'localhost:27018'], replicaset='foo', ...)

Read full details here:

- http://api.mongodb.com/python/current/examples/high_availability.html#connecting-to-a-replica-set


#database #mongodb #mongo #replica_set #replication #pymongo #arbiter #master #primary #slave

Backup and Restor from couchDB

- https://github.com/danielebailo/couchdb-dump


#database #couchdb #couch #couchBase #backup

If you want to make an exact copy of a table from another database into a target database in mySQL you could do like below:

create table new_table like target_database.target_table

The above command will create a table named new_table like target_table from target_database database.

#mysql #database #create_table #table #copy_table

https://medium.com/mongoaudit/how-to-enable-tls-ssl-on-mongodb-d973a92cefa6

#database #mongodb #mongo #tls #ssl #lets_encrypt

Turn MySQL table into utf8mb4 to store emojis:

ALTER TABLE YOUR_TABLE convert to character set utf8mb4 collate utf8mb4_general_ci;

Moreover you also need to change column character set:

ALTER TABLE YOUR_TABLE CHANGE YOUR_COLUMN_NAME YOUR_COLUMN_NAME  VARCHAR(250) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;

Be careful that now you have to do more things like set character set after connection initiation in Python:

your_mysql_client = MySQLdb.connect(...)
 your_mysql_client.set_character_set('utf8mb4')

Now before executing your query you also need to set character set on cursor:

my_cursor.execute("SET NAMES utf8mb4;")
 my_cursor.execute(YOUR_QUERY)

#database #mysql #character_set #utf8mb4 #cursor #emoji

With mysqldump you can export databases. with --port parameter you can specify which port it should connects. If you provide localhost for --host parameter, mySQL will use sockets and port will be ignored.

So be careful with it!

#mysql #mysqldump #port #port_ignorance #3306 #backup #database_backup #sockets #ip_address #localhost

Table compression in MySQL:

https://dev.mysql.com/doc/refman/5.5/en/innodb-compression-usage.html

#database #mysql #compression #innodb

https://www.jetbrains.com/research/devecosystem-2018/databases/

It seems MongoDB is trying to be the first in the next decade.

#database #survey

How do you query is not null in MongoDB?

db.mycollection.find({ 'fieldname' : { $ne: null } });

#database #mongodb #query #ne #null #not_null

How to clone a database in MySQL?

mysqldump -u root db_name | mysql -u root new_db_name

NOTE: if it gets password provide -p to both commands.

#mysql #clone #copy #database #copy_database

What does select_related do in Django?

select_related does a join in case needed on the DB side and reduce query counts. Let's look at an example:

# Hits the database.
e = Entry.objects.get(id=5)

# Hits the database again to get the related Blog object.
b = e.blog

In the above code 2 queries are issued in DB side. First it gets Entry record and then blog is fetched from DB when e.blog is called. And here’s select_related lookup:

# Hits the database.
e = Entry.objects.select_related('blog').get(id=5)

# Doesn't hit the database, because e.blog has been prepopulated
# in the previous query.
b = e.blog

You can follow foreign keys in a similar way to querying them. If you have the following models:

from django.db import models

class City(models.Model):
    # ...
    pass

class Person(models.Model):
    # ...
    hometown = models.ForeignKey(
        City,
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
    )

class Book(models.Model):
    # ...
    author = models.ForeignKey(Person, on_delete=models.CASCADE)

Then a call to Book.objects.select_related('author__hometown').get(id=4) will cache the related Person and the related City:

# Hits the database with joins to the author and hometown tables.
b = Book.objects.select_related('author__hometown').get(id=4)
p = b.author         # Doesn't hit the database.
c = p.hometown       # Doesn't hit the database.

# Without select_related()...
b = Book.objects.get(id=4)  # Hits the database.
p = b.author         # Hits the database.
c = p.hometown       # Hits the database.

#python #django #select_related #join #database #models

https://www.eventstore.com/eventstoredb

#database #grpc #event #event_store

Did you know you can use jsonSchema in MongoDB to search for documents?

Let's say you have users collection with data below:

{ "_id" : ObjectId("5f64bd1eca8806f2c04fcbe3"), "customer_id" : 100, "username" : "john" }
{ "_id" : ObjectId("5f64bd1eca8806f2c04fcbe5"), "customer_id" : 206, "username" : "new_customer" }
{ "_id" : ObjectId("60420df441558d6671cf54f2"), "customer_id" : "123", "username" : "Ali" }

Now let's say you want to find all documents that has a customer_id of type string instead of int.
In Mongo shell:

let ms = {required: ["customer_id"], properties: {customer_id: {bsonType: "string"}}}

This schema says look for documents that have customer_id field with string type. To search:

> db.customers.find({$jsonSchema: ms})
{ "_id" : ObjectId("60420df441558d6671cf54f2"), "customer_id" : "123", "username" : "Ali" }

Interesting, right? :)

#database #mongodb #jsonSchema #json_schema