12. Advanced searching - Django Database queries¶
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
… who are in Rooms tagged as
… and who has the Attribute
lycantrophywith 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 had a subclass
Cannon and we only wanted the cannons, we would do
all_cannons = Cannon.objects.all()
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:
really_all_weapons = Weapon.objects.all_family()
This result now contains both
Cannon instances (and any other
entities whose typeclasses inherit at any distance from
To limit your search by other criteria than the Typeclass you need to use
roses = Flower.objects.filter(db_key="rose")
This is a queryset representing all flowers having a
db_key equal to
Since this is a queryset you can keep adding to it; this will act as an
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.
Note how we use
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
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, but search all subclasses as well.
get, but search all subclasses as well.
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
.filterquery to the result of
evennia.search_objectto further refine the search.
12.1. Queryset field lookups¶
Above we found roses with exactly the
"rose". This is an exact match that is case sensitive,
so it would not find
# 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 not allowed in a function keyword.
roses = Flower.objects.filter(db_key__icontains="rose")
This will find all flowers whose name contains the string
"wild rose" etc. The
i in the beginning makes the search case-insensitive. Other useful variations to use
__iendswith. You can also use
__ge for “greater-than”/“greater-or-equal-than” comparisons (same for
__le). There is also
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:
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:
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
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.
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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 ) )
Line 4 We want to find
Characters, so we access
We start to filter …
Line 6: … by accessing the
db_locationfield (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
db_keyis “monlit” (non-case sensitive).
Line 7: … We also want only Characters with
Line 8 :… at the same time as the
db_valueis 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
(“we want tag room to be “monlit” and lycantrhopy be > 2”).
For queries using
NOT we need Django’s Q object. It is imported from Django directly:
from django.db.models import Q
Q is an object that is created with the same arguments as
.filter, for example
You can then use this
Q instance as argument in a
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):
AND. A tilde
~ in front negates the expression inside the
Q and thus
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
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() )
This reads as “Find all Characters in a moonlit room that either has the
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.
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)
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
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:
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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) )
Count is a Django class for counting the number of things in the database.
Line 6-7: Here we first create an annotation
Count. It creates an in-database function that will count the number of results inside the database. The fact annotation means that now
num_objectsis avaiable to be used in other parts of the query.
Line 8 We filter on this annotation, using the name
num_objectsas something we can filter for. We use
num_objects__gte=5which means that
num_objectsshould 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.
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_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.
.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:
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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
idis unique) we find for each organization and store it in a ‘variable’
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_listmethod 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) ]
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.