12. Advanced searching - Django Database queries

Important

More advanced lesson!

Learning about Django’s query language is very useful once you start doing more advanced things in Evennia. But it’s not strictly needed out the box and can be a little overwhelming for a first reading. So if you are new to Python and Evennia, feel free to just skim this lesson and refer back to it later when you’ve gained more experience.

The search functions and methods we used in the previous lesson are enough for most cases. But sometimes you need to be more specific:

• You want to find all Characters …

• … who are in Rooms tagged as moonlit …

• … and who has the Attribute lycantrophy with a level higher than 2 …

• … because they should immediately transform to werewolves!

In principle you could achieve this with the existing search functions combined with a lot of loops and if statements. But for something non-standard like this, querying the database directly will be much more efficient.

Evennia uses Django to handle its connection to the database. A django queryset represents a database query. One can add querysets together to build ever-more complicated queries. Only when you are trying to use the results of the queryset will it actually call the database.

The normal way to build a queryset is to define what class of entity you want to search by getting its .objects resource, and then call various methods on that. We’ve seen variants of this before:

all_weapons = Weapon.objects.all()

This is now a queryset representing all instances of Weapon. If Weapon had a subclass Cannon and we only wanted the cannons, we would do

all_cannons = Cannon.objects.all()

Note that Weapon and Cannon are different typeclasses. This means that you won’t find any Weapon-typeclassed results in all_cannons. Vice-versa, you won’t find any Cannon-typeclassed results in all_weapons. This may not be what you expect.

If you want to get all entities with typeclass Weapon as well as all the subclasses of Weapon, such as Cannon, you need to use the _family type of query:

_family

The all_family and filter_family (as well as get_family for getting exactly one result) are Evennia-specific. They are not part of regular Django.

really_all_weapons = Weapon.objects.all_family()

This result now contains both Weapon and Cannon instances (and any other entities whose typeclasses inherit at any distance from Weapon, like Musket or Sword).

To limit your search by other criteria than the Typeclass you need to use .filter (or .filter_family) instead:

roses = Flower.objects.filter(db_key="rose")

This is a queryset representing all flowers having a db_key equal to "rose". Since this is a queryset you can keep adding to it; this will act as an AND condition.

local_roses = roses.filter(db_location=myroom)

We could also have written this in one statement:

local_roses = Flower.objects.filter(db_key="rose", db_location=myroom)

We can also .exclude something from results

local_non_red_roses = local_roses.exclude(db_key="red_rose")

It’s important to note that we haven’t called the database yet! Not until we actually try to examine the result will the database be called. Here the database is called when we try to loop over it (because now we need to actually get results out of it to be able to loop):

for rose in local_non_red_roses:
    print(rose)

From now on, the queryset is evaluated and we can’t keep adding more queries to it - we’d need to create a new queryset if we wanted to find some other result. Other ways to evaluate the queryset is to print it, convert it to a list with list() and otherwise try to access its results.

database fields

Each database table have only a few fields. For DefaultObject, the most common ones are db_key, db_location and db_destination. When accessing them they are normally accessed just as obj.key, obj.location and obj.destination. You only need to remember the db_ when using them in database queries. The object description, obj.db.desc is not such a hard-coded field, but one of many Attributes attached to the Object.

Note how we use db_key and db_location. This is the actual names of these database fields. By convention Evennia uses db_ in front of every database field. When you use the normal Evennia search helpers and objects you can skip the db_ but here we are calling the database directly and need to use the ‘real’ names.

Here are the most commonly used methods to use with the objects managers:

• filter - query for a listing of objects based on search criteria. Gives empty queryset if none were found.

• get - query for a single match - raises exception if none were found, or more than one was found.

• all - get all instances of the particular type.

• filter_family - like filter, but search all subclasses as well.

• get_family - like get, but search all subclasses as well.

• all_family - like all, but return entities of all subclasses as well.

All of Evennia search functions use querysets under the hood. The evennia.search_* functions actually return querysets (we have just been treating them as lists so far). This means you could in principle add a .filter query to the result of evennia.search_object to further refine the search.

12.1. Queryset field lookups

Above we found roses with exactly the db_key "rose". This is an exact match that is case sensitive, so it would not find "Rose".

# this is case-sensitive and the same as =
roses = Flower.objects.filter(db_key__exact="rose"

# the i means it's case-insensitive
roses = Flower.objects.filter(db_key__iexact="rose")

The Django field query language uses __ similarly to how Python uses . to access resources. This is because . is not allowed in a function keyword.

roses = Flower.objects.filter(db_key__icontains="rose")

This will find all flowers whose name contains the string "rose", like "roses", "wild rose" etc. The i in the beginning makes the search case-insensitive. Other useful variations to use are __istartswith and __iendswith. You can also use __gt, __ge for “greater-than”/“greater-or-equal-than” comparisons (same for __lt and __le). There is also __in:

swords = Weapons.objects.filter(db_key__in=("rapier", "two-hander", "shortsword"))

One also uses __ to access foreign objects like Tags. Let’s for example assume this is how we have identified mages:

char.tags.add("mage", category="profession")

Now, in this case we already have an Evennia helper to do this search:

mages = evennia.search_tags("mage", category="profession")

Here is what it would look as a query if you were only looking for Vampire mages:

Breaking lines of code

In Python you can wrap code in (...) to break it over multiple lines. Doing this doesn’t affect functionality, but can make it easier to read.

sparkly_mages = (
	Vampire.objects.filter(									   
           db_tags__db_key="mage", 
           db_tags__db_category="profession")
    )

This looks at the db_tags field on the Vampire and filters on the values of each tag’s db_key and db_category together.

For more field lookups, see the django docs on the subject.

12.2. Let’s get that werewolf …

Let’s see if we can make a query for the werewolves in the moonlight we mentioned at the beginning of this lesson.

Firstly, we make ourselves and our current location match the criteria, so we can test:

> py here.tags.add("moonlit")
> py me.db.lycantrophy = 3

This is an example of a more complex query. We’ll consider it an example of what is possible.

from typeclasses.characters import Character

will_transform = (
    Character.objects
    .filter(
        db_location__db_tags__db_key__iexact="moonlit",
        db_attributes__db_key="lycantrophy",
        db_attributes__db_value__gt=2
    )
)

Attributes vs database fields

Don’t confuse database fields with Attributes you set via obj.db.attr = 'foo' or obj.attributes.add(). Attributes are custom database entities linked to an object. They are not separate fields on that object like db_key or db_location are.

• Line 4 We want to find Characters, so we access .objects on the Character typeclass.

• We start to filter …

  • Line 6: … by accessing the db_location field (usually this is a Room)

    • … and on that location, we get the value of db_tags (this is a many-to-many database field that we can treat like an object for this purpose; it references all Tags on the location)

    • … and from those Tags, we looking for Tags whose db_key is “monlit” (non-case sensitive).

  • Line 7: … We also want only Characters with Attributes whose db_key is exactly "lycantrophy"

  • Line 8 :… at the same time as the Attribute’s db_value is greater-than 2.

Running this query makes our newly lycantrophic Character appear in will_transform so we know to transform it. Success!

12.3. Queries with OR or NOT

All examples so far used AND relations. The arguments to .filter are added together with AND (“we want tag room to be “monlit” and lycantrhopy be > 2”).

For queries using OR and NOT we need Django’s Q object. It is imported from Django directly:

from django.db.models import Q

The Q is an object that is created with the same arguments as .filter, for example

Q(db_key="foo")

You can then use this Q instance as argument in a filter:

q1 = Q(db_key="foo")
Character.objects.filter(q1)
# this is the same as 
Character.objects.filter(db_key="foo")

The useful thing about Q is that these objects can be chained together with special symbols (bit operators): | for OR and & for AND. A tilde ~ in front negates the expression inside the Q and thus works like NOT.

q1 = Q(db_key="Dalton")
q2 = Q(db_location=prison)
Character.objects.filter(q1 | ~q2)

Would get all Characters that are either named “Dalton” or which is not in prison. The result is a mix of Daltons and non-prisoners.

Let us expand our original werewolf query. Not only do we want to find all Characters in a moonlit room with a certain level of lycanthrophy - we decide that if they have been newly bitten, they should also turn, regardless of their lycantrophy level (more dramatic that way!).

Let’s say that getting bitten means that you’ll get assigned a Tag recently_bitten.

This is how we’d change our query:

from django.db.models import Q

will_transform = (
    Character.objects
    .filter(
        Q(db_location__db_tags__db_key__iexact="moonlit")
        & (
          Q(db_attributes__db_key="lycantrophy",
            db_attributes__db_value__gt=2)
          | Q(db_tags__db_key__iexact="recently_bitten")
        ))
    .distinct()
)

That’s quite compact. It may be easier to see what’s going on if written this way:

from django.db.models import Q

q_moonlit = Q(db_location__db_tags__db_key__iexact="moonlit")
q_lycantropic = Q(db_attributes__db_key="lycantrophy", db_attributes__db_value__gt=2)
q_recently_bitten = Q(db_tags__db_key__iexact="recently_bitten")

will_transform = (
    Character.objects
    .filter(q_moonlit & (q_lycantropic | q_recently_bitten))
    .distinct()
)

SQL

These Python structures are internally converted to SQL, the native language of the database. If you are familiar with SQL, these are many-to-many tables joined with LEFT OUTER JOIN, which may lead to multiple merged rows combining the same object with different relations.

This reads as “Find all Characters in a moonlit room that either has the Attribute lycantrophy higher than two, or which has the Tag recently_bitten”. With an OR-query like this it’s possible to find the same Character via different paths, so we add .distinct() at the end. This makes sure that there is only one instance of each Character in the result.

12.4. Annotations

What if we wanted to filter on some condition that isn’t represented easily by a field on the object? An example would wanting to find rooms only containing five or more objects.

We could do it like this (don’t actually do it this way!):

from typeclasses.rooms import Room

  all_rooms = Rooms.objects.all()

  rooms_with_five_objects = []
  for room in all_rooms:
      if len(room.contents) >= 5:
          rooms_with_five_objects.append(room)

list.append, extend and .pop

Use mylist.append(obj) to add new items to a list. Use mylist.extend(another_list)) or list1 + list2 to merge two lists together. Use mylist.pop() to remove an item from the end or .pop(0) to remove from the beginning of the list. Remember all indices start from 0 in Python.

Above we get all rooms and then use list.append() to keep adding the right rooms to an ever-growing list. This is not a good idea, once your database grows this will be unnecessarily compute-intensive. It’s much better to query the database directly

Annotations allow you to set a ‘variable’ inside the query that you can then access from other parts of the query. Let’s do the same example as before directly in the database:

from typeclasses.rooms import Room
from django.db.models import Count

rooms = (
    Room.objects
    .annotate(
        num_objects=Count('locations_set'))
    .filter(num_objects__gte=5)
)

locations_set

Note the use of locations_set in that Count. The *s_set is a back-reference automatically created by Django. In this case it allows you to find all objects that has the current object as location.

Count is a Django class for counting the number of things in the database.

• Line 6-7: Here we first create an annotation num_objects of type Count. It creates an in-database function that will count the number of results inside the database. The fact annotation means that now num_objects is avaiable to be used in other parts of the query.

• Line 8 We filter on this annotation, using the name num_objects as something we can filter for. We use num_objects__gte=5 which means that num_objects should be greater than or equal to 5.

Annotations can be a little harder to get one’s head around but much more efficient than lopping over all objects in Python.

12.5. F-objects

What if we wanted to compare two dynamic parameters against one another in a query? For example, what if instead of having 5 or more objects, we only wanted objects that had a bigger inventory than they had tags (silly example, but …)?

This can be with Django’s F objects. So-called F expressions allow you to do a query that looks at a value of each object in the database.

from django.db.models import Count, F
from typeclasses.rooms import Room

result = (
    Room.objects
    .annotate(
        num_objects=Count('locations_set'),
        num_tags=Count('db_tags'))
    .filter(num_objects__gt=F('num_tags'))
)

Here we used .annotate to create two in-query ‘variables’ num_objects and num_tags. We then directly use these results in the filter. Using F() allows for also the right-hand-side of the filter condition to be calculated on the fly, completely within the database.

12.6. Grouping and returning only certain properties

Suppose you used tags to mark someone belonging to an organization. Now you want to make a list and need to get the membership count of every organization all at once.

The .annotate, .values_list, and .order_by queryset methods are useful for this. Normally when you run a .filter, what you get back is a bunch of full typeclass instances, like roses or swords. Using .values_list you can instead choose to only get back certain properties on objects. The .order_by method finally allows for sorting the results according to some criterion:

from django.db.models import Count
from typeclasses.rooms import Room

result = (
    Character.objects
    .filter(db_tags__db_category="organization")
    .annotate(tagcount=Count('id'))
    .order_by('-tagcount'))
    .values_list('db_tags__db_key', "tagcount")

Here we fetch all Characters who …

• Line 6: … has a tag of category “organization” on them

• Line 7:… along the way we count how many different Characters (each id is unique) we find for each organization and store it in a ‘variable’ tagcount using .annotate and Count

• Line 8: … we use this count to sort the result in descending order of tagcount (descending because there is a minus sign, default is increasing order but we want the most popular organization to be first).

• Line 9: … and finally we make sure to only return exactly the properties we want, namely the name of the organization tag and how many matches we found for that organization. For this we use the values_list method on the queryset. This will evaluate the queryset immediately.

The result will be a list of tuples ordered in descending order by the number of matches, in a format like the following:

[
 ('Griatch's poets society', 3872),
 ("Chainsol's Ainneve Testers", 2076),
 ("Blaufeuer's Whitespace Fixers", 1903),
 ("Volund's Bikeshed Design Crew", 1764),
 ("Tehom's Glorious Misanthropes", 1763)
]

12.7. Conclusions

We have covered a lot of ground in this lesson and covered several more complex topics. Knowing how to query using Django is a powerful skill to have.