Note: This is considered an advanced topic.
Evennia offers many convenient ways to store object data, such as via Attributes or Scripts. This is sufficient for most use cases. But if you aim to build a large stand-alone system, trying to squeeze your storage requirements into those may be more complex than you bargain for. Examples may be to store guild data for guild members to be able to change, tracking the flow of money across a game-wide economic system or implement other custom game systems that requires the storage of custom data in a quickly accessible way.
Overview of database tables¶
SQL-type databases (which is what Evennia supports) are basically highly optimized systems for retrieving text stored in tables. A table may look like this
id | db_key | db_typeclass_path | db_permissions ... ------------------------------------------------------------------ 1 | Griatch | evennia.DefaultCharacter | Developers ... 2 | Rock | evennia.DefaultObject | None ...
Each line is considerably longer in your database. Each column is referred to as a “field” and every row is a separate object. You can check this out for yourself. If you use the default sqlite3 database, go to your game folder and run
You will drop into the database shell. While there, try:
sqlite> .help # view help sqlite> .tables # view all tables # show the table field names for objects_objectdb sqlite> .schema objects_objectdb # show the first row from the objects_objectdb table sqlite> select * from objects_objectdb limit 1; sqlite> .exit
Evennia uses Django, which abstracts away the database SQL manipulation and allows you to search and manipulate your database entirely in Python. Each database table is in Django represented by a class commonly called a model since it describes the look of the table. In Evennia, Objects, Scripts, Channels etc are examples of Django models that we then extend and build on.
Adding a new database table¶
Here is how you add your own database table/models:
In Django lingo, we will create a new “application” - a subsystem under the main Evennia program. For this example we’ll call it “myapp”. Run the following (you need to have a working Evennia running before you do this, so make sure you have run the steps in [Setup Quickstart](Getting- Started) first):
evennia startapp myapp mv myapp world (linux) move myapp world (windows)
A new folder
myappis created. “myapp” will also be the name (the “app label”) from now on. We move it into the
world/subfolder here, but you could keep it in the root of your
mygameif that makes more sense. 1. The
myappfolder contains a few empty default files. What we are interested in for now is
models.pyyou define your model(s). Each model will be a table in the database. See the next section and don’t continue until you have added the models you want.
You now need to tell Evennia that the models of your app should be a part of your database scheme. Add this line to your
mygame/server/conf/settings.pyfile (make sure to use the path where you put
myappand don’t forget the comma at the end of the tuple):
INSTALLED_APPS = INSTALLED_APPS + ("world.myapp", )
evennia makemigrations myapp evennia migrate myapp
This will add your new database table to the database. If you have put your game under version control (if not, you should), don’t forget to
git add myapp/* to add all items
to version control.
Defining your models¶
A Django model is the Python representation of a database table. It can be handled like any other Python class. It defines fields on itself, objects of a special type. These become the “columns” of the database table. Finally, you create new instances of the model to add new rows to the database.
We won’t describe all aspects of Django models here, for that we refer to the vast Django documentation on the subject. Here is a (very) brief example:
from django.db import models class MyDataStore(models.Model): "A simple model for storing some data" db_key = models.CharField(max_length=80, db_index=True) db_category = models.CharField(max_length=80, null=True, blank=True) db_text = models.TextField(null=True, blank=True) # we need this one if we want to be # able to store this in an Evennia Attribute! db_date_created = models.DateTimeField('date created', editable=False, auto_now_add=True, db_index=True)
We create four fields: two character fields of limited length and one text field which has no maximum length. Finally we create a field containing the current time of us creating this object.
db_date_createdfield, with exactly this name, is required if you want to be able to store instances of your custom model in an Evennia Attribute. It will automatically be set upon creation and can after that not be changed. Having this field will allow you to do e.g.
obj.db.myinstance = mydatastore. If you know you’ll never store your model instances in Attributes the
db_date_createdfield is optional.
You don’t have to start field names with
db_, this is an Evennia convention. It’s nevertheless recommended that you do use
db_, partly for clarity and consistency with Evennia (if you ever want to share your code) and partly for the case of you later deciding to use Evennia’s
SharedMemoryModel parent down the line.
The field keyword
db_index creates a database index for this field, which allows quicker lookups, so it’s recommended to put it on fields you know you’ll often use in queries. The
blank=True keywords means that these fields may be left empty or set to the empty string without the database complaining. There are many other field types and keywords to define them, see django docs for more info.
Similar to using django-admin you are able to do
evennia inspectdb to get an automated listing of model information for an existing database. As is the case with any model generating tool you should only use this as a starting
point for your models.
Referencing existing models and typeclasses¶
You may want to use
ManyToManyField to relate your new model to existing ones.
To do this we need to specify the app-path for the root object type we want to store as a string (we must use a string rather than the class directly or you’ll run into problems with models not having been initialized yet).
"objects.ObjectDB"for all Objects (like exits, rooms, characters etc)
"comms.Msg"for Msg objects.
"help.HelpEntry"for Help Entries.
Here’s an example:
from django.db import models class MySpecial(models.Model): db_character = models.ForeignKey("objects.ObjectDB") db_items = models.ManyToManyField("objects.ObjectDB") db_account = modeles.ForeignKey("accounts.AccountDB")
It may seem counter-intuitive, but this will work correctly:
myspecial.db_character = my_character # a Character instance my_character = myspecial.db_character # still a Character
This works because when the
.db_character field is loaded into Python, the entity itself knows that it’s supposed to be a
Character and loads itself to that form.
The drawback of this is that the database won’t enforce the type of object you store in the relation. This is the price we pay for many of the other advantages of the Typeclass system.
db_character field fail if you try to store an
Account, it will gladly accept any instance of a typeclass that inherits from
ObjectDB, such as rooms, exits or other non-character Objects. It’s up to you to validate that what you store is what you expect it to be.
Creating a new model instance¶
To create a new row in your table, you instantiate the model and then call its
from evennia.myapp import MyDataStore new_datastore = MyDataStore(db_key="LargeSword", db_category="weapons", db_text="This is a huge weapon!") # this is required to actually create the row in the database! new_datastore.save()
Note that the
db_date_created field of the model is not specified. Its flag
at_now_add=True makes sure to set it to the current date when the object is created (it can also not be changed further after creation).
When you update an existing object with some new field value, remember that you have to save the object afterwards, otherwise the database will not update:
my_datastore.db_key = "Larger Sword" my_datastore.save()
Evennia’s normal models don’t need to explicitly save, since they are based on
SharedMemoryModel rather than the raw django model. This is covered in the next section.
Searching for your models¶
To search your new custom database table you need to use its database manager to build a query. Note that even if you use
SharedMemoryModel as described in the previous section, you have to use the actual field names in the query, not the wrapper name (so
db_key and not just
from world.myapp import MyDataStore # get all datastore objects exactly matching a given key matches = MyDataStore.objects.filter(db_key="Larger Sword") # get all datastore objects with a key containing "sword" # and having the category "weapons" (both ignoring upper/lower case) matches2 = MyDataStore.objects.filter(db_key__icontains="sword", db_category__iequals="weapons") # show the matching data (e.g. inside a command) for match in matches2: self.caller.msg(match.db_text)
See the Beginner Tutorial lesson on Django querying for a lot more information about querying the database.