Dump Django tables and restore em in data-migration form

migration1.py

# coding: utf-8
from __future__ import unicode_literals
from django.db import migrations, models

# FUNCTION TO DUMP DATA
def backup_tables(apps, schema_editor):
    from django.core import serializers

    Campaign = apps.get_model('reminders', 'Campaign')
    CampaignSnapshot = apps.get_model('reminders', 'CampaignSnapshot')

    with open('reminders/fixtures/campaigns.json', 'w+') as file:
        file.write(serializers.serialize('json', Campaign.objects.all()))
    with open('reminders/fixtures/snapshots.json', 'w+') as file:
        file.write(serializers.serialize('json', CampaignSnapshot.objects.all()))


class Migration(migrations.Migration):

    dependencies = [
        ('reminders', '0004_auto_20180914_1813'),
    ]

    operations = [
        migrations.RunPython(backup_tables),  # <<<<<< ----- USAGE OF THAT FUNCTION BEFORE ANY ACTIONS. Added RunPython in operations.
        migrations.RemoveField(
            model_name='campaign',
            name='client',
        ),
      #... here goes the rest of migration that is ommited due it was generated automatically

migration3.py

### This migration created manyally as a "blank migration":
### ./manage.py makemigrations reminders --empty

# coding: utf-8
from __future__ import unicode_literals
from django.db import migrations, models

# RESTORE FUNCTION THAT IMPLEMENTS CORRECT LOADING OF DATA INTO CHANGED SCHEMA 
def restore_data(apps, schema_editor):
    from datetime import datetime
    from decimal import Decimal
    import json

    Client = apps.get_model('reminders', 'Client')
    Campaign = apps.get_model('reminders', 'Campaign')
    CampaignSnapshot = apps.get_model('reminders', 'CampaignSnapshot')

    with open('reminders/fixtures/campaigns.json', 'r') as campaigns_file:
        campaigns_data = json.loads(campaigns_file.read())
        for campaign in campaigns_data:
            client = Client.objects.get(id=campaign['fields']['client'])
            Campaign.objects.create(client=client,
                                    direct_id=campaign['pk'],
                                    name=campaign['fields']['name'],
                                    archived=campaign['fields']['archived'])

    with open('reminders/fixtures/snapshots.json', 'r') as snapshots_file:
        snapshots_data = json.loads(snapshots_file.read())
        for snapshot in snapshots_data:
            campaign = Campaign.objects.get(direct_id=snapshot['fields']['campaign'])
            CampaignSnapshot.objects.create(campaign=campaign,
                                            sum=Decimal(snapshot['fields']['sum']),
                                            created=datetime.strptime(snapshot['fields']['created'], '%Y-%m-%d').date())


class Migration(migrations.Migration):

    dependencies = [
        ('reminders', '0006_auto_20181005_1646'),
    ]

    operations = [
        migrations.RunPython(restore_data)
    ]