1. ORM是什么

ORM 是 python編程語言后端web框架 Django的核心思想，“Object Relational Mapping”，即對象-關(guān)系映射，簡稱ORM。

一個(gè)句話理解就是：創(chuàng)建一個(gè)實(shí)例對象，用創(chuàng)建它的類名當(dāng)做數(shù)據(jù)表名，用創(chuàng)建它的類屬性對應(yīng)數(shù)據(jù)表的字段，當(dāng)對這個(gè)實(shí)例對象操作時(shí)，能夠?qū)?yīng)MySQL語句

demo:

class User(父類省略):uid = ('uid', "int unsigned")name = ('username', "varchar(30)")email = ('email', "varchar(30)")password = ('password', "varchar(30)")...省略...u = User(uid=12345, name='Michael', email='test@orm.org', password='my-pwd') u.save() # 對應(yīng)如下sql語句 # insert into User (username,email,password,uid) # values ('Michael','test@orm.org','my-pwd',12345)

說明

所謂的ORM就是讓開發(fā)者在操作數(shù)據(jù)庫的時(shí)候，能夠像操作對象時(shí)通過xxxx.屬性=yyyy一樣簡單，這是開發(fā)ORM的初衷

只不過ORM的實(shí)現(xiàn)較為復(fù)雜，Django中已經(jīng)實(shí)現(xiàn)了 很復(fù)雜的操作，本節(jié)知識(shí) 主要通過完成一個(gè) insert相類似的ORM，理解其中的道理就就可以了

2. 通過元類簡單實(shí)現(xiàn)ORM中的insert功能

class ModelMetaclass(type):def __new__(cls, name, bases, attrs):mappings = dict()# 判斷是否需要保存for k, v in attrs.items():# 判斷是否是指定的StringField或者IntegerField的實(shí)例對象if isinstance(v, tuple):print('Found mapping: %s ==> %s' % (k, v))mappings[k] = v# 刪除這些已經(jīng)在字典中存儲(chǔ)的屬性for k in mappings.keys():attrs.pop(k)# 將之前的uid/name/email/password以及對應(yīng)的對象引用、類名字attrs['__mappings__'] = mappings # 保存屬性和列的映射關(guān)系attrs['__table__'] = name # 假設(shè)表名和類名一致return type.__new__(cls, name, bases, attrs)class User(metaclass=ModelMetaclass):uid = ('uid', "int unsigned")name = ('username', "varchar(30)")email = ('email', "varchar(30)")password = ('password', "varchar(30)")# 當(dāng)指定元類之后，以上的類屬性將不在類中，而是在__mappings__屬性指定的字典中存儲(chǔ)# 以上User類中有 # __mappings__ = {# "uid": ('uid', "int unsigned")# "name": ('username', "varchar(30)")# "email": ('email', "varchar(30)")# "password": ('password', "varchar(30)")# }# __table__ = "User"def __init__(self, **kwargs):for name, value in kwargs.items():setattr(self, name, value)def save(self):fields = []args = []for k, v in self.__mappings__.items():fields.append(v[0])args.append(getattr(self, k, None))sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join([str(i) for i in args]))print('SQL: %s' % sql)u = User(uid=12345, name='Michael', email='test@orm.org', password='my-pwd') # print(u.__dict__) u.save()

執(zhí)行的效果:

Found mapping: password ==> ('password', 'varchar(30)') Found mapping: email ==> ('email', 'varchar(30)') Found mapping: uid ==> ('uid', 'int unsigned') Found mapping: name ==> ('username', 'varchar(30)') SQL: insert into User (uid,password,username,email) values (12345,my-pwd,Michael,test@orm.org)

3. 完善對數(shù)據(jù)類型的檢測

class ModelMetaclass(type):def __new__(cls, name, bases, attrs):mappings = dict()# 判斷是否需要保存for k, v in attrs.items():# 判斷是否是指定的StringField或者IntegerField的實(shí)例對象if isinstance(v, tuple):print('Found mapping: %s ==> %s' % (k, v))mappings[k] = v# 刪除這些已經(jīng)在字典中存儲(chǔ)的屬性for k in mappings.keys():attrs.pop(k)# 將之前的uid/name/email/password以及對應(yīng)的對象引用、類名字attrs['__mappings__'] = mappings # 保存屬性和列的映射關(guān)系attrs['__table__'] = name # 假設(shè)表名和類名一致return type.__new__(cls, name, bases, attrs)class User(metaclass=ModelMetaclass):uid = ('uid', "int unsigned")name = ('username', "varchar(30)")email = ('email', "varchar(30)")password = ('password', "varchar(30)")# 當(dāng)指定元類之后，以上的類屬性將不在類中，而是在__mappings__屬性指定的字典中存儲(chǔ)# 以上User類中有 # __mappings__ = {# "uid": ('uid', "int unsigned")# "name": ('username', "varchar(30)")# "email": ('email', "varchar(30)")# "password": ('password', "varchar(30)")# }# __table__ = "User"def __init__(self, **kwargs):for name, value in kwargs.items():setattr(self, name, value)def save(self):fields = []args = []for k, v in self.__mappings__.items():fields.append(v[0])args.append(getattr(self, k, None))args_temp = list()for temp in args:# 判斷入如果是數(shù)字類型if isinstance(temp, int):args_temp.append(str(temp))elif isinstance(temp, str):args_temp.append("""'%s'""" % temp)sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join(args_temp))print('SQL: %s' % sql)u = User(uid=12345, name='Michael', email='test@orm.org', password='my-pwd') # print(u.__dict__) u.save()

運(yùn)行效果如下:

Found mapping: uid ==> ('uid', 'int unsigned') Found mapping: password ==> ('password', 'varchar(30)') Found mapping: name ==> ('username', 'varchar(30)') Found mapping: email ==> ('email', 'varchar(30)') SQL: insert into User (email,uid,password,username) values ('test@orm.org',12345,'my-pwd','Michael')

4. 抽取到基類中

class ModelMetaclass(type):def __new__(cls, name, bases, attrs):mappings = dict()# 判斷是否需要保存for k, v in attrs.items():# 判斷是否是指定的StringField或者IntegerField的實(shí)例對象if isinstance(v, tuple):print('Found mapping: %s ==> %s' % (k, v))mappings[k] = v# 刪除這些已經(jīng)在字典中存儲(chǔ)的屬性for k in mappings.keys():attrs.pop(k)# 將之前的uid/name/email/password以及對應(yīng)的對象引用、類名字attrs['__mappings__'] = mappings # 保存屬性和列的映射關(guān)系attrs['__table__'] = name # 假設(shè)表名和類名一致return type.__new__(cls, name, bases, attrs)class Model(object, metaclass=ModelMetaclass):def __init__(self, **kwargs):for name, value in kwargs.items():setattr(self, name, value)def save(self):fields = []args = []for k, v in self.__mappings__.items():fields.append(v[0])args.append(getattr(self, k, None))args_temp = list()for temp in args:# 判斷入如果是數(shù)字類型if isinstance(temp, int):args_temp.append(str(temp))elif isinstance(temp, str):args_temp.append("""'%s'""" % temp)sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join(args_temp))print('SQL: %s' % sql)class User(Model):uid = ('uid', "int unsigned")name = ('username', "varchar(30)")email = ('email', "varchar(30)")password = ('password', "varchar(30)")u = User(uid=12345, name='Michael', email='test@orm.org', password='my-pwd') # print(u.__dict__) u.save()

總結(jié)

以上是生活随笔為你收集整理的Django中的核心思想ORM---元类实现ORM的全部內(nèi)容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網(wǎng)站內(nèi)容還不錯(cuò)，歡迎將生活随笔推薦給好友。