django notes 六：数据库 CRUD 操作

# create
b = Blog(name=‘Beatles Blog‘, tagline=‘All the latest Beatles news.‘)
b.save()

# create and save
Blog.objects.create(name=‘Beatles Blog‘, tagline=‘All the latest Beatles news.‘)

# update, 外键字段也是一样的
b5.name = ‘New name‘
b5.save()

# 添加 ManyToManyField 用 add
joe = Author.objects.create(name="Joe")
entry.authors.add(joe)

# delete
e.delete()
Entry.objects.filter(pub_date__year=2005).delete()

# 查询 QuerySet
# 默认都是 lazy query, 用到时才会真正执行数据库查询

# 打印生成的 sql 语句, 直接访问 QuerySet 实例的 query 属性
t = Tag.objects.all()
print t.query

# get all
all_entries = Entry.objects.all()

# 查询字段  field + 双下环线 + 操作的类型
Entry.objects.all().filter(pub_date__year=2006)
Entry.objects.filter(pub_date__lte=‘2006-01-01‘)

# 取 1 条数据，取不到会抛  DoesNotExist 异常，取到多条时会抛 MultipleObjectsReturned 异常
one_entry = Entry.objects.get(pk=1)

# slice
Entry.objects.all()[:5]  # 转成 sql 是 LIMIT 5

# 外键字段引用
Entry.objects.filter(blog_id=4)
Entry.objects.filter(blog__name=‘Beatles Blog‘)

# F expressions 引用并操作数据库字段
from django.db.models import F
Entry.objects.filter(n_comments__gt=F(‘n_pingbacks‘))
Entry.objects.filter(n_comments__gt=F(‘n_pingbacks‘) * 2)

# 主键查询
Blog.objects.get(id=14) # __exact is implied
Blog.objects.get(pk=14) # pk implies id__exact

# Q object 执行数据库 and  or  操作
Q(question__startswith=‘Who‘) | Q(question__startswith=‘What‘) # WHERE question LIKE ‘Who%‘ OR question LIKE ‘What%‘
Q(question__startswith=‘Who‘) & Q(name=‘leslie‘)

# order_by 默认升序，- 表示降序
Entry.objects.filter(pub_date__year=2005).order_by(‘-pub_date‘, ‘headline‘)

# Manager.raw() 方法执行 raw sql
for p in Person.objects.raw(‘SELECT * FROM myapp_person‘):
    print(p)