Peewee和数据库继承

Question

我正在学习Peewee和Bottle，想做一个书籍笔记应用。

假设我有以下这些实体：

Subject
Book
Chapter
Note
Tag

我希望能为章节、书籍和主题创建笔记。

在数据库中，你可以这样做：

create table noteable (
    noteable_id INT AUTO_INCREMENT PRIMARY KEY
    ,type VARCHAR(10) NOT NULL CHECK (type in ('SUBJECT','BOOK','CHAPTER','NOTE'))
);
create table subject (
    subject_id INT AUTO_INCREMENT PRIMARY KEY
    ,noteable_id INT UNIQUE REFERENCES noteable (noteable_id)
    ,...
);
create table book (
    book_id INT AUTO_INCREMENT PRIMARY KEY
    ,subject_id INT NOT NULL REFERENCES subject (subject_id)
    ,noteable_id INT UNIQUE REFERENCES noteable (noteable_id)
    ,...
);
create table chapter(
    chapter_id INT AUTO_INCREMENT PRIMARY KEY
    ,book_id INT NOT NULL REFERENCES book (book_id)
    ,noteable_id INT UNIQUE REFERENCES noteable(noteable_id)
    ,...
);

create table note(
    note_id INT AUTO_INCREMENT PRIMARY KEY
    ,noteable_id INT UNIQUE REFERENCES noteable(noteable_id)
    ,...
);

（如果你想让笔记和可记笔记的对象之间有多对多的关系，你还需要做一个叫note_notable的桥接表）。

你会在主题、书籍和章节上设置插入前触发器，这样在插入新数据时就会在noteable表中插入一行数据，然后获取新插入行的noteable_id，并把这个ID用在即将插入的行上。

我假设如果你使用像Peewee这样的ORM，你会想在应用程序逻辑中处理这些，而不是依赖触发器。

我该如何在Peewee中实现这个模型呢？

Answer 1

这是我实现的方法。我在Peewee中找不到原生的继承实现方式，所以我自己动手做了。如果有更好的方法，请告诉我，我会给你奖励。

import MySQLdb
import peewee
from peewee import *
from datetime import datetime

db = MySQLDatabase('test', user='root',passwd='psswd')

class BaseModel(Model):
    class Meta:
        database = db

class Noteable(BaseModel):
    type = CharField(null = False)

# This will act as the trigger that inserts a row into noteable,
# and retrieves the notable.id to use
class N(BaseModel):
    def save(self, *args, **kwargs):
        if not self.id:
            noteable = Noteable(type=self.__class__.__name__.upper())
            noteable.save()
            self.noteable = noteable.id
        return super(N, self).save(*args, **kwargs)

class Subject(N):
    name = CharField(null = False, unique = True)
    noteable = ForeignKeyField(Noteable, related_name="noteablesubject", null= False, unique = True)


class Book(N):
    name = CharField(null = False, unique = True)
    subject = ForeignKeyField(Subject, related_name="books", null = False)
    noteable = ForeignKeyField(Noteable, related_name="noteablebook", null= False, unique = True)


class Chapter(N):
    name = CharField(null = False)
    chapter_number = IntegerField(null = False)
    book = ForeignKeyField(Book, related_name="chapters")
    noteable = ForeignKeyField(Noteable, related_name="noteablechapter", null= False, unique = True)


class Note(BaseModel):
    note = TextField(null = False)
    # N.B. unique is not true, as multiple notes can go to the same subject/book/chapter
    noteable = ForeignKeyField(Noteable, related_name="notes", null= False)


Note.drop_table(True)
Chapter.drop_table(True)
Book.drop_table(True)
Subject.drop_table(True)
Noteable.drop_table(True)

Noteable.create_table(True)
Subject.create_table(True)
Book.create_table(True)
Chapter.create_table(True)
Note.create_table(True)

s = Subject(name="subject")
s.save()
n = Note(note="subject notes", noteable = s.noteable)
n.save()
n = Note(note="subject notes 2", noteable = s.noteable)
n.save()
b = Book(name="book", subject=s)
b.save()
n = Note(note="book notes", noteable = b.noteable)
n.save()
n = Note(note="book notes 2", noteable = b.noteable)
n.save()
c = Chapter(chapter_number=1, name="chapter", book=b)
c.save()
n = Note(note="chapter notes", noteable=c.noteable)
n.save()
n = Note(note="chapter notes 2", noteable=c.noteable)
n.save()

（如果你想在笔记和著名人物之间建立多对多的关系，你需要定义一个NoteNotable类，并添加外键，同时从Note中移除外键）

你可以定义一个辅助方法，来左连接任意类和笔记：

def get_notes(clazz, id):
    return clazz.select().join(Noteable).join(Note, JOIN_LEFT_OUTER).where(clazz.id = id)

你可以像这样遍历它：

% for note in chapter.noteable.notes:

% end

这是从 SELECT * FROM NOTABLE; 查询得到的结果：

+----+---------+
| id | type    |
+----+---------+
|  1 | SUBJECT |
|  2 | BOOK    |
|  3 | CHAPTER |
+----+---------+

这是从 SELECT * FROM NOTE; 查询得到的结果：

+----+-----------------+-------------+
| id | note            | noteable_id |
+----+-----------------+-------------+
|  1 | subject notes   |           1 |
|  2 | subject notes 2 |           1 |
|  3 | book notes      |           2 |
|  4 | book notes 2    |           2 |
|  5 | chapter notes   |           3 |
|  6 | chapter notes 2 |           3 |
+----+-----------------+-------------+