Django database operation and problem solving

add to

#  surface .objects.create

#  Method 1 
a = User.objects.create(userNum=123, name='ABB', age=18, sex=' male ')

#  Method 2 
d = dict(userNum=123, name='ABB', age=18, sex=' male ')
a = User.objects.create(**d)

# Method 3 
a = User(userNum=123, name='ABB', age=18, sex=' male ')
a.save()

Parameters	explain
get_or_create	As long as one field value is different from that in the data table （ Divide primary key ）, Will perform the insertion operation If they are identical, the insertion operation is not performed , But the data object that returns this row of data
update_or_create	Judge whether the current data exists in the data table , If it exists, update , Otherwise, it is new table data
bulk_create	Batch operation of data v1 = User(userNum=123, name=‘ABB’, age=18, sex=‘ male ’) v2 = User(userNum=124, name=‘ABC’, age=19, sex=‘ Woman ’) info_list = [v1,v2] User.objects.bulk_create(info_list)

Delete

#  surface .objects.filter().delete()

#  Method 1   Delete all content 
User.objects.all().delete()

# Method 2   Delete one name by ABC The data of 
User.objects.filter(name='ABC').delete()

# Method 3   Delete multiple data 
User.objects.filter(age=18).delete()

In the process of deletion, the data has a foreign key field , The data associated with the foreign key will be deleted at the same time , Delete mode reference models.py Set in the PROTECT、SET_NULL etc.

modify

#  surface .objects.filter().update()

#  Method 1   modify name by ABC The gender of is gay
User.objects.filter(name='ABC').update(sex='gay')

#  Method 2  
a = dict(age='1')
User.objects.filter(name='ABC').update(**a)

#  Method 3   Use F Method to realize self increment / Self reduction 
from djanto.db.models import F
User.objects.filter(name='ABC').update(age=F('age')+1)

Inquire about

# select * from user  A full table query 
a = User.objects.all()
#  Check the first 
a[0].name

# select * from user LIMIT3  Check before 3 strip 
a = User.objects.all()[:3]

# filter  You can also add multiple conditions 
User.objects.filter(name='ABC',age=18)

# SQL in or Method  select * from user where name='ABC' or age=18 , Need to introduce  Q
from django.db.models import Q
User.objects.filter(Q(name='ABC') | Q(age=18))

# SQL in not Method  select * from user where not name='ABC' , stay Q Before to add ~
User.objects.filter(~Q(name='ABC'))

#  Statistical quantity 
User.objects.filter(name='ABC').count()

#  duplicate removal  select DISTINCT name from user where name='ABC' ,distinct There is no need to set parameters , The weight removal method is based on values
a = User.objects.values('name').filter(name='ABC').distinct()

#  Descending sort query , In descending order order_by Set in the  '-'
User.objects.order_by('-age')

Match symbol	Use	explain
__exact	filter(name__exact=‘ABC’)	It's exactly the same as
__iexact	filter(name__iexact =‘ABC’)	Exactly equal to and ignore case
__contains	filter(name__contains =‘ABC’)	Fuzzy matching , similar SQL in like %ABC%
__icontains	filter(name__icontains =‘ABC’)	Blur match and ignore case
__gt	filter(name__gt =1)	Greater than
__gte	filter(name__gte =1)	Greater than or equal to
__lt	filter(name__lt=1)	Less than
__lte	filter(name__lte=1)	Less than or equal to
__isnull	filter(name__isnull=True/False)	Determine whether it is null

Associated table foreign key parameters on_delete

(1)、on_delete = None：
 When deleting the data of the associated table , Current table and associated table filed act .
(2)、on_delete = models.CASCADE：
 Indicates cascade delete , When associated table （ Sub table ） When deleting data in , The corresponding foreign key （ Parent table ） Also delete the data in .
(3)、on_delete = models.DO_NOTHING:
 You delete your , father （ Foreign keys ） I don't want to care about you 
(4)、on_delete = models.PROTECT:
 Protected mode , If this method is adopted , When the associated data is deleted, an ProtectError error 
(5)、on_delete = models.SET_DEFAULT:
 Set the default value , When deleting sub table fields , The foreign key field is set to the default value , So when defining a foreign key, you should add a default value .
(6)、on_delete = models.SET（ value ）:
 When deleting associated data , Customize a value , The value can only correspond to the specified entity 

Database fields

• character string

  password=models.CharField(verbose_name=' password ：',max_length=50)
  

• Set primary key

  account=models.CharField(verbose_name=' account number ：',max_length=50,primary_key=True,db_index=True)
  db_index: Add index 
  

• Set the associated foreign key

  account=models.ForeignKey(User_Account,on_delete = models.CASCADE)
  

• integer

  group_id=models.IntegerField(verbose_name=' Group chat number ')
  

• Boolean type

  models.BooleanField(verbose_name=' Whether you can add files ',default=False)
  

• The radio

  #sex choices  Set radio , The value before tuple is true, which is the stored value , The following values are display values sex=models.CharField(max_length=1,choices=[(' male ',' male '),(' Woman ',' Woman ')])
  

• Time

  DateTimeField(DateField)x
   date + Time format  YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ]
  
  models.DateField(verbose_name='token Update time ',auto_now=True)
   Date format  YYYY-MM-DD
  auto_now： Each modification is automatically updated 
  auto_now_add： The data creation time is the time when the data is added , Subsequent modification of other field data will not change 
  
  TimeField(DateTimeCheckMixin, Field)
   Time format  HH:MM[:ss[.uuuuuu]]
  

clear database , Rebuild the table

• Want to delete the database model you practiced before

  Order.objects.all().values().delete()
  

• Delete file

  • Delete all tables in the database
  • Delete migrations All in folder file , except __init__.py file
  • function

  python manage.py makemigrations
  python manage.py migrate
  

django solve manage.py migrate Invalid question

• Solution

  python manage.py dbshell  Into the database , perform delete from django_migrations where app='your_appname';
  
  python manage.py makemigrations( if migrations File not deleted , Don't do this step )
  
  python manage.py migrate  All right. , Be accomplished 
  

• reason

   Cause multiple applications migrations The reason for the failure is , At present model It was modified , The original migrations It has been deleted by me , however , Regenerated migrations Use incrementing integers to name , therefore , stay django_migrations In the table 0001,0002 And so on, the files of the first few numbers have been recorded , stay Django It seems , Being recorded is equivalent to being applied , therefore , It's going to start with No migrations to apply.