Django multiple database foreign keys

I am trying to implement multiple database support for my django (version 1.11) app. For that purpose I have included in my settings.py:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': 'labrin_dbase',
        'USER': 'labrin_admin',
        'PASSWORD': 'ndzwwZHv63STuvAF?C_$L@j@*@epZXaX',
        'HOST': 'localhost',
        'PORT': '5432',
    },
    'comment': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': 'labrin_dbase_comments',
        'USER': 'labrin_admin_comments',
        'PASSWORD': 'adzwaTHv63STuvAF!C_$L@j@*@epZXaY',
        'HOST': 'localhost',
        'PORT': '5433',
    }
}

DATABASE_ROUTERS = [
    'labrin_task.comment_router.CommentRouter',
]

And my database router is configured as below:

class CommentRouter(object):
    def db_for_read(self, model, **hints):
        if model._meta.db_table == 'todo_comment':
            return 'comment'
        return None

    def db_for_write(self, model, **hints):
        if model._meta.db_table == 'todo_comment':
            return 'comment'
        return None

    def allow_relation(self, obj1, obj2, **hints):
        return True

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        if model_name == 'comment':
            return db == 'comment'
        return None

Models in my "todo" app(which is only app in project):

from django.db import models
from django.contrib.auth import get_user_model

UserModel = get_user_model()


class Todo(models.Model):
    name = models.CharField(max_length=64)
    description = models.TextField()
    author = models.ForeignKey(UserModel, on_delete=models.CASCADE)
    deadline = models.DateTimeField()
    created_at = models.DateTimeField(auto_now_add=True)


class Comment(models.Model):
    todo = models.ForeignKey(Todo, on_delete=models.CASCADE)
    author = models.ForeignKey(UserModel, on_delete=models.CASCADE)
    text = models.CharField(max_length=256)
    created_at = models.DateTimeField(auto_now_add=True)


class ShareTodo(models.Model):
    todo = models.ForeignKey(Todo, on_delete=models.CASCADE)
    with_user = models.ForeignKey(UserModel, on_delete=models.CASCADE)
    comment_allowed = models.BooleanField(default=False)

When I remove comment database and DATABASE_ROUTERS from settings.py, my app is working normally. After adding mentioned to settings.py, my app returns an error when I create Comment object. The error says:

Exception inside application: insert or update on table "todo_comment" violates foreign key constraint "todo_comment_author_id_bb272a3e_fk_auth_user_id" DETAIL: Key (author_id)=(1) is not present in table "auth_user". What am I doing wrong?

Note: I am starting two postgres servers as separate docker containers and after running containers, I run python manage.py migrate and python manage.py migrate --database=comment for making all migrations.