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Clean up the last part of the beginner tutorial part 1

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docs/source/Howtos/Beginner-Tutorial/Part1/Django-queries.md
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@ -2,10 +2,11 @@
```{important} More advanced lesson!
Learning about Django's queryset 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.
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.
@ -14,7 +15,7 @@ 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'll should immediately transform to werewolves!
- ... 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
@ -35,27 +36,34 @@ only wanted the cannons, we would do
all_cannons = Cannon.objects.all()
Note that `Weapon` and `Cannon` are different typeclasses. You won't find any `Cannon` instances in
the `all_weapon` result above, confusing as that may sound. To get instances of a Typeclass _and_ the
instances of all its children classes you need to use `_family`:
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:
```{sidebar} _family
The all_family, filter_family etc is an Evennia-specific
thing. It's not part of regular Django.
The all_family, filter_family etc is an Evennia-specific
thing. It's not part of regular Django.
```
really_all_weapons = Weapon.objects.all_family()
This result now contains both `Weapon` and `Cannon` instances.
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`
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 objects having a `db_key` equal to `"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)
@ -68,8 +76,10 @@ We can also `.exclude` something from results
local_non_red_roses = local_roses.exclude(db_key="red_rose")
Only until we actually try to examine the result will the database be called. Here it's called when we
try to loop over the queryset:
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)
@ -78,9 +88,21 @@ From now on, the queryset is _evaluated_ and we can't keep adding more queries t
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_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.
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.
```{sidebar} database fields
Each database table have only a few fields. For `Objects`, 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
arbitrary Attributes attached to the Object.
```
Here are the most commonly used methods to use with the `objects` managers:
@ -108,7 +130,7 @@ so it would not find `"Rose"`.
# the i means it's case-insensitive
roses = Flower.objects.filter(db_key__iexact="rose")
The Django field query language uses `__` in the same way as Python uses `.` to access resources. This
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")
@ -120,7 +142,8 @@ 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 identify mages:
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")
@ -141,7 +164,7 @@ For more field lookups, see the
## 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 section.
of this lesson.
Firstly, we make ourselves and our current location match the criteria, so we can test:
@ -153,9 +176,9 @@ possible.
```{sidebar} Line breaks
Note the way of writing this code. It would have been very hard to read if we just wrote it in
one long line. But since we wrapped it in `(...)` we can spread it out over multiple lines
without worrying about line breaks!
Note the way of writing this code. It would have been very hard to read if we
just wrote it in one long line. But since we wrapped it in `(...)` we can spread
it out over multiple lines without worrying about line breaks!
```
```python
@ -166,21 +189,23 @@ will_transform = (
.filter(
db_location__db_tags__db_key__iexact="moonlit",
db_attributes__db_key="lycantrophy",
db_attributes__db_value__gt=2)
db_attributes__db_value__gt=2
)
)
```
- **Line 3** - We want to find `Character`s, so we access `.objects` on the `Character` typeclass.
- **Line 4** - We start to filter ...
- **Line 5**
- We want to find `Character`s, so we access `.objects` on the `Character` typeclass.
- We start to filter ...
-
- ... 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 6** - ... We also want only Characters with `Attributes` whose `db_key` is exactly `"lycantrophy"`
- **Line 7** - ... at the same time as the `Attribute`'s `db_value` is greater-than 2.
- ... We also want only Characters with `Attributes` whose `db_key` is exactly `"lycantrophy"`
- ... at the same time as the `Attribute`'s `db_value` is greater-than 2.
Running this query makes our newly lycantrrophic Character appear in `will_transform`. Success!
Running this query makes our newly lycantrophic Character appear in `will_transform` so we
know to transform it. Success!
> Don't confuse database fields with [Attributes](../../../Components/Attributes.md) you set via `obj.db.attr = 'foo'` or
`obj.attributes.add()`. Attributes are custom database entities *linked* to an object. They are not
@ -218,10 +243,12 @@ works like `NOT`.
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`. Now we also want the full moon to immediately transform people who were
recently bitten, even if their `lycantrophy` level is not yet high enough (more dramatic this way!). Let's say there is
a Tag "recently_bitten" that controls this.
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`. Now we also
want the full moon to immediately transform people who were recently bitten,
even if their `lycantrophy` level is not yet high enough (more dramatic this
way!). When you get bitten, you'll get a Tag `recently_bitten` put on you to
indicate this.
This is how we'd change our query:
@ -259,21 +286,24 @@ will_transform = (
```{sidebar} 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.
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.
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.
## Annotations
What if we wanted to filter on some condition that isn't represented easily by a field on the
object? Maybe we want to find rooms only containing five or more objects?
What if we wanted to filter on some condition that isn't represented easily by a
field on the object? Maybe we want to find rooms only containing five or more
objects?
We *could* do it like this (don't actually do it this way!):
@ -288,12 +318,14 @@ We *could* do it like this (don't actually do it this way!):
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 computing-intensive. The database is much more suitable for this.
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 computing-intensive. The database is much more
suitable for this.
_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:
_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:
```python
from typeclasses.rooms import Room
@ -315,17 +347,19 @@ that will count the number of results inside the database.
> Note the use of `location_set` in that `Count`. The `*_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*.
Next 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 5. This is a little
harder to get one's head around but much more efficient than lopping over all objects in Python.
Next 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. This is a little harder to get one's head
around but much more efficient than lopping over all objects in Python.
## 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](https://docs.djangoproject.com/en/1.11/ref/models/expressions/#f-expressions).
So-called F expressions allow you to do a query that looks at a value of each object in the database.
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](https://docs.djangoproject.com/en/1.11/ref/models/expressions/#f-expressions).
So-called F expressions allow you to do a query that looks at a value of each
object in the database.
```python
from django.db.models import Count, F
@ -390,8 +424,9 @@ in a format like the following:
## 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.
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.
This concludes the first part of the Evennia starting tutorial - "What we have". Now we have a good foundation
to understand how to plan what our tutorial game will be about.
This concludes the first part of the Evennia starting tutorial - "What we have".
Now we have a good foundation to understand how to plan what our tutorial game
will be about.