Example of usage Django and Citus.

1. Install Citus

   https://omidraha.com/en/latest/src/citus/tips.html#install-citus-on-multi-machine-cluster-on-ubuntu

2. Run this command on master and each one of workers:

    $ createdb citus_exp

    $ psql -d citus_exp -c "CREATE EXTENSION citus;"

3. Register workers on master for this citus_exp db

    $ psql -d citus_exp -c "SELECT * from master_add_node('w1', 5432);"
    $ psql -d citus_exp -c "SELECT * from master_add_node('w2', 5432);"

4. Verify:

    $ psql -d citus_exp -c "SELECT * FROM master_get_active_worker_nodes()";

Output:

 node_name | node_port
-----------+-----------
 w1        |      5432
 w2        |      5432

Errors

1. python manage.py migrate

django.db.utils.DataError: replication_factor (1) exceeds number of worker nodes (0)
HINT:  Add more worker nodes or try again with a lower replication factor.

2. "SELECT create_distributed_table('ads_ads','id')"

psycopg2.OperationalError: cannot create reference table "ads_ads"
DETAIL:  There are no active worker nodes.

3. "SELECT create_distributed_table('ads_ads','id')"

django.db.utils.ProgrammingError: cannot create foreign key constraint
DETAIL:  Referenced table must be a distributed table.

4. "SELECT create_reference_table('ads_ads')"

django.db.utils.NotSupportedError: cannot create foreign key constraint because reference tables are not supported as the referencing table of a foreign constraint
DETAIL:  Reference tables are only supported as the referenced table of a foreign key when the referencing table is a hash distributed table

citusdata/citus#2378

Currently Citus supports foreign keys in two cases;

From hash distributed tables to hash distributed tables when the foreign key includes the distribution column,
From hash distributed tables to reference tables on any column.

5. "SELECT create_distributed_table('ads_campaign', 'company_id')"

django.db.utils.NotSupportedError: cannot create constraint on "ads_campaign"
DETAIL:  Distributed relations cannot have UNIQUE, EXCLUDE, or PRIMARY KEY constraints that do not include the partition column (with an equality operator if EXCLUDE).

Example models:

class Company(models.Model):
    name = models.CharField(max_length=512)
    users = models.ManyToManyField(User)
    created_at = models.DateTimeField(auto_created=True)
    updated_at = models.DateTimeField(auto_now_add=True)


class Campaign(models.Model):
    class Meta:
        unique_together = (('id', 'company'),)

    name = models.CharField(max_length=512)
    company = models.ForeignKey(Company, on_delete=models.CASCADE)
    created_at = models.DateTimeField(auto_created=True)
    updated_at = models.DateTimeField(auto_now_add=True)

Because there is a builtin id field on the Campaign model, that is a primary key field.

6. When you want to add user to company , while company has one many to many field to user:

user = User.objects.first()
now = timezome.now()
company = Company.objects.create(name='c1', created_at=now)
company.users.add(user)

InternalError: consistency check on SPI tuple count failed
CONTEXT:  SQL statement "SELECT 1 FROM ONLY "public"."ads_company" x WHERE "id" OPERATOR(pg_catalog.=) $1 FOR KEY SHARE OF x"

Example model:

class Company(models.Model):
    name = models.CharField(max_length=512)
    users = models.ManyToManyField(User)
    created_at = models.DateTimeField(auto_created=True)
    updated_at = models.DateTimeField(auto_now_add=True)