ORM’s *add, delete, modify, and check* operations on the database in Django

Foreword

What is an ORM?

ORM (Object Relational Mapping) refers to using object-oriented methods to handle operations such as creating tables in the database and adding, deleting, modifying, and querying data.

A table in the database = a class.

Each record in the database = an object.

In short, defining a class in Django is to create a table in the database. Instantiating an object of a class in Django adds a record to the database. Deleting an object in Django deletes a record in the database. Changing the properties of an object in DJango is to modify the value of a record in the database. Traversing the attribute values ​​of the query object in Django is to query the value of the record in the database.

The following are several command statements in Django’s views function.

One, increase (create, save)

from app01.models import *

     #Create method one: Author.objects.create(name=\'Alvin\')

     #Create method two: Author.objects.create(**{"name":"alex"})

     #save method one: author=Author(name="alvin")
  author.save()

     #Save method two: author=Author()
                     author.name="alvin"
                     author.save()

Two, delete

>>> Book.objects.filter(id=1).delete()
 (3, {\'app01.Book_authors\': 2, ​​\'app01.Book\': 1})

 If it is a many-to-many relationship: remove() and clear() methods:

 #Forward
 book = models.Book.objects.filter(id=1)

 #Delete all related information associated with girl 1 in the third table
 book.author.clear() #Clear all data associated with id=1 in book
 book.author.remove(2) #can be id
 book.author.remove(*[1,2,3,4]) #Can be a list, add * in front

 #Reverse
 author = models.Author.objects.filter(id=1)
 author.book_set.clear() #Clear all data associated with id=1 in boy

Three, change (update and save)

#------------- The update method directly sets the corresponding attributes-------------  ---
     models.Book.objects.filter(id=3).update(title="PHP")
     ##sql:
     ##UPDATE "app01_book" SET "title" = \'PHP\' WHERE "app01_book"."id" = 3; args=(\'PHP\', 3)


 #--------------- The save method will reset all attributes, which is inefficient----------  
     obj=models.Book.objects.filter(id=3)[0]
     obj.title="Python"
     obj.save()
 # SELECT "app01_book"."id", "app01_book"."title", "app01_book"."price", 
 # "app01_book"."color", "app01_book"."page_num", 
 # "app01_book"."publisher_id" FROM "app01_book" WHERE "app01_book"."id" = 3 LIMIT 1; 
 # 
 # UPDATE "app01_book" SET "title" = \'Python\', "price" = 3333, "color" = \'red\', "page_num" = 556,
 # "publisher_id" = 1 WHERE "app01_book"."id" = 3;

Four, check (filter and get)

# Query related APIs: 

 # filter(**kwargs): It contains objects that match the given filter conditions

 # all(): Query all results

 # get(**kwargs): Returns objects that match the given filtering conditions. There is and only one result returned. If there are more than one objects that meet the filtering conditions or there are none, it will not happen.  Throw an error.  

 #-----------The following methods are all for processing the query results: such as objects.filter.values()-------  -

 # values(*field): Returns a ValueQuerySet - a special QuerySet. What you get after running is not a series of model instantiated objects, but an iterable  dictionary sequence
                                     
 # exclude(**kwargs): It contains objects that do not match the given filter conditions

 # order_by(*field): Sort query results

 # reverse(): Sort query results in reverse order

 # distinct(): Remove duplicate records from the returned results

 # values_list(*field): It is very similar to values(). It returns a sequence of tuples, and values ​​returns a sequence of dictionaries

 # count(): Returns the number of objects matching the query (QuerySet) in the database.  

 # first(): Return the first record

 # last(): Returns the last record

 # exists(): If the QuerySet contains data, it returns True, otherwise it returns False.