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Migrating (dummy) content into a Django site

Project description

Tivol

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Welcome to Tivol. You probably wonder to your self "what's this Django app do?". Let's start with a scenario: you created your Django site, or a backend with REST framework and you start to insert data which you work with. After a while two things came up:

  1. Someone new joined your team and the new guy has no data to work with
  2. You set up a CI with selenium and you need to run e2e tests which means you need to create data during the tests

There are more scenarios but all of them comes to the same point - you need real life data.

This is where tivol come handy, you can keep data in source files and insert them to the DB thus allow you to work on dummy content without any problem.

We'll cover up later on how to develop features in your project with a DOD (Data Oriented Development) approach but first, let's see how to integrated your Django project with Tivol.

Setup

First, we need to register the map. We can do this by adding it to the installed app list:

INSTALLED_APPS = [
    ...
    'tivol',
]

After that, we'll need to register a "Migration entry point". Migration entry point tells to the tivol package the migration handlers we create. The migration handlers will eventually take data from a source file, or a folder, and will insert them to the DB.

First, you'll need to create an entry point class:

from tivol.base_classes.entry_point import EntryPoint


class CustomEntryPoint(EntryPoint):

    def register_migrations(self):
        pass

It's recommended to create a folder, i.e tivol_migration, and holds there the data and the migrations handlers (which will be covered later on).

After we created out custom entry point, you'll need to register it like that:

TIVOL_ENTRY_POINT = 'path.to.the.CustomEntryPoint'

Migrate content

After registering the entry point, we need to introduce our data files to the Tivol application. There are two steps for this process. This process will repeat it self each time you want to add more content migrations.

Register migration handlers

First, we need register the migration handler. Remember the entry point you created earlier? Awesome, go there. You can register the migration handler by using the method add_migration_handler. It suppose to look like this:

from dummyapp.tivol_migrations.animals_migration import AnimalMigration
from dummyapp.tivol_migrations.companies_migration import CompanyMigration
from tivol.base_classes.entry_point import EntryPoint


class CustomEntryPoint(EntryPoint):

    def register_migrations(self):
        self.add_migration_handler(AnimalMigration)
        self.add_migration_handler(CompanyMigration)

Notice that we only register the class reference and not instantiating it.

The class we're referencing will provide information for Tivol about the migration: where is the source of our data, which source mapper will handle the data and much more. It's also provides for us migration life cycle hooks - before content migration, after content migration end much more. We'll discuss it in the future.

How to write a migration source

Well... there is no actual rule except for one thing: each row, collection of values we want to import, must have an ID. This used for not importing the same data twice and to have to ability to rollback the migrated content from the DB. For more examples - have a look here

Writing source mapper

Source mapper is something that take data from one place and then return a list dictionaries which then can be inserted to the DB using Django's ORM (but this part is not your responsibility). Let's look on the Yaml source mapper, since it's the smallest one:

class YamlMapper(BaseMapper):

    def process_single(self, file):
        return yaml.load(file, Loader=yaml.FullLoader)

The only logic that relate for processing data from a place and return it is the process_single method. That method will be invoke in case of a single file or a directory. No need to worry about how we opened the file, that someone else's problem, just keep in mind that the method receives a file object need to return a list of dictionaries which represent the rows in the file.

Writing migration handler

This is where the magic happens. We going to inspect the class we registered as a migration handler. Let's look first on the code:

class AnimalMigrations(MigrationHandlerBase):

    def init_metadata(self):
        self.id = 'animal'
        self.name = 'Animal migration'
        self.description = 'Migrating animals into the system'

        csv_mapper = CsvMapper()
        csv_mapper.set_destination_file(path=os.path.join(os.getcwd(), 'dummyapp', 'tivol_migrations', 'source_files', 'animals.csv'))
        self.add_source_mapper(csv_mapper)

        self.set_model_target(Animal)

The migration handler extends from the MigrationHandlerBase. For the basic migration workflow we need to use the init_metadata, as you already saw, and there's a couple of code section that we need to discuss about:

self.id = 'animal'
self.name = 'Animal migration'
self.description = 'Migrating animals into the system'

In this part we described the migration and what's it going to do. Please notice that's there's an ID property. That property will help us track which migration handler migrated which content. You should keep it and in plural format. On the other hand... it's really that important so you can write there any string you'ld like to(Emoji have not been tested yet)

csv_mapper = CsvMapper()
csv_mapper.set_destination_file(path=os.path.join(os.getcwd(), 'dummyapp', 'tivol_migrations', 'source_files', 'animals.csv'))
self.add_source_mapper(csv_mapper)

In this part we created an instance of the CsvMapper, specified the path of the CSV file and registered it. Tivol need this one so we could get data from the file(s) and insert them to the DB.

The last is the, self.set_model_target(Animal) which tells Tivol what is the DB model object. Again, don't pass the instantiated object but the reference to the object.

Alter source data

There are couple of ways to alter source data. But first - why? Well, we can have a lot of reasons: changing a date string to a date object, split a string into a list of other models in the DB and reference it to the DB records which going to be inserted ino the DB. There could be various ways:

Process plugins

First, let's look how to register a plugin. We'll take an example of two plugins. In the init_metadata method we'll add the next section:

self.fields_plugins = {
    'name': [UppercasePlugin],
    'founded_at': [{'plugin': DatePlugin, 'extra_info': {'format': '%B %d, %Y'}}]
}

The fields_plugins property is a key-value which goes by the rules that the key is the property from the source, and the field in the DB, and the value is a list of plugins which will take the data from the source file and apply logic that transform it to something else.

The value is a list of referenced classes, like the name or maybe a list of dictionaries which describe what's the plugin that will be invoke, in this case the plugin key in the dictionary, and the extra_info is a dictionary which will be passed as dictionary to the process method in the plugin which in our case will be the format of the string that represent a date.

Now, let's look at a plugin - the date process plugin:

class DatePlugin(PluginBase):
    """
    Getting a string and transform it to a string.
    """

    def process(self, value, extra_info=None):
        return datetime.strptime(value, extra_info['format'])

The plugin is pretty easy to understand - the value argument is the value from the source file and the extra_info argument represent a list of values, such as the format date.

Reference basing on migrated records

For example, we need to migrate directors and movies. We also need to keep a relationship between a movie and the director of that movie. Let's look on two CSV files:

directors.csv:

id,name
director_1,Michael Benjamin Bay
director_2,Martin Scorsese

Now, how should movies.csv look like? like that:

id,name,director
movie_1,The Wolf of Wall Street,director_2
movie_2,The Wolf of Wall Street,director_2
movie_3,The Departed,director_2
movie_4,Pearl Harbor,director_1
movie_5,Transformers,director_1
movie_5,Transformers 2: Revenge of the Fallen,director_1

The next part is to set the reference plugin like this:

self.fields_plugins = {
    'director': [{'plugin': ReferencePlugin, 'extra_info': {'model': Director}}],
}

How the magic works? Tivol keeps track of the ID from the source files, CSV, JSON or DB records, and know what is the ID of the record in the DB after the migration process completed. The reference plugin returns the object as Django's ORM expect it to be.

Migration life cycle hooks

TBD

Database migration

We can pull data from other databases. For now, MySQL but more will be available in the future. Migrations which relies on data from files are pretty easy to set up - tell the mapper where the file store and the mapper will do the heavy lifting. But how data from other databases can be migrated easily without a lot of hustle? Well, Django already has a nice DB layer which we can use. Let's see how this will work.

First, we need set the DB connection. In your settings.py, or local_settings.py, you'll need to add connections to the DB. Django's documentation has a lot of information for that but you can have a look on the next example:

DATABASES = {
    'default': {
        # ...
    },
    'other_site': {
        'ENGINE': 'django.db.backends.mysql',
        'NAME': 'django_migration',
        'USER': 'root',
        'PASSWORD': 'root',
        'HOST': 'localhost',
        'PORT': '3306',
    }
}

After we set up the connection, let's see how to connect the mapper. In the init_metadata method we need to configure the mapper like this:

        mysql_mapper = SqlMapper()
        mysql_mapper.set_connection('other_site')
        mysql_mapper.set_table('tags')

Those three lines did a lot for us: initialised the SqlMapper instance. The mysql_mapper.set_connection('other_site') told to the mapper which connection to use and the mysql_mapper.set_table('tags') told the mapper from which table in another DB we need to pull the DB.

Tivol CLI commands

Let's go over some CLI commands we get out of the box:

Migrate content

So you create content and now you to import it in? No problem. Just hit:

python manage.py migrate_content

You'll get something like this:

Start to migrate
 1/2 [■■■■■■■■■■■■■■--------------]  50% Animal migration: 7 migrated, 0 skipped
 2/2 [■■■■■■■■■■■■■■■■■■■■■■■■■■■■] 100% Company migration: 5 migrated, 0 skipped
Migrated

Get migration information

You can get information about the migration we have in the system:

python3.6 manage.py migrations_info

This will return a table which look like this:

Migration name Number of items Number of migrated items
Animal migration 7 7
Company migration 6 5

Rollback migration

There's could be a couple of reasons for rolling back the data: someone changed the values of the migrated values and things not working properly or just you want to clean you DB from the dummy content.

Type the next command:

python3.6 manage.py migrations_rollback

You'll see a nice progress bar how the procedure going:

Are you sure you want to remove any migrated data? (yes/no) [yes]
Starting to rollback migration. Collecting migrated rows
  1/13 [■■--------------------------]   7% Removing Animal:1
  2/13 [■■■■------------------------]  15% Removing Animal:2
  3/13 [■■■■■■----------------------]  23% Removing Animal:3
  4/13 [■■■■■■■■--------------------]  30% Removing Animal:4
  5/13 [■■■■■■■■■■------------------]  38% Removing Animal:5
  6/13 [■■■■■■■■■■■■----------------]  46% Removing Animal:6
  7/13 [■■■■■■■■■■■■■■■-------------]  53% Removing Animal:7
  8/13 [■■■■■■■■■■■■■■■■■-----------]  61% Removing Company:1
  9/13 [■■■■■■■■■■■■■■■■■■■---------]  69% Removing Company:2
 10/13 [■■■■■■■■■■■■■■■■■■■■■-------]  76% Removing Company:3
 11/13 [■■■■■■■■■■■■■■■■■■■■■■■-----]  84% Removing Company:4
 12/13 [■■■■■■■■■■■■■■■■■■■■■■■■■---]  92% Removing Company:5
 13/13 [■■■■■■■■■■■■■■■■■■■■■■■■■■■■] 100% Removing Company:6

Extra info

If you want to look at some examples or some blog post look the next list:

  • Dummy app - holds examples for the feature Tivol has to offer

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