CSIC_716_2020109 [Django Getting Started --- models commonly used in the field, key parameters in the field, database query optimization, open transactions]

Common Fields

AutoField (primary_key = True) the primary key field 

as CharField (MAX_LENGTH = 32) VARCHAR (32) 

IntegerField () int 

BigIntergerField () BIGINT 

DecimalField () decimal 

EmailField () VARCHART (254) 

the DateField () DATE 

DateTimeField () datetime 
	auto_now: per edit when data is automatically update the field time 
	auto_now_add: automatically updated when the data is created 
BooleanField (field) 

	into the field will pass Boolean value corresponds to a digital 0/1 
	is_delete 
	is_status 
	is_vip 

the TextField (field,) 
    - text type 
	store large pieces of text 

the FileField ( field,) 
    - string stored in the path database, file uploading to the specified directory, file path only keep 
	upload_to = 'specify a file path' 
	to the field transfer file object file is automatically saved to the specified folder upload_to then save the file field route of

　　

How to customize the char type field

Import django.db.models Field, from 


class RealCharField (Field,): 
    DEF the __init __ (Self, MAX_LENGTH, args *, ** kwargs): 
        self.max_length MAX_LENGTH # = intercept a parent class using a method of operation after completing the super call the parent class method 
        Super () .__ the init __ (MAX_LENGTH = MAX_LENGTH, args *, ** kwargs) 

    DEF db_type (Self, Connection): 
        return 'char (% S)'% self.max_length

　　

 

parameter choices

Enumeration is similar to:
define a large range tuple gender_choice representative may choose large number of small sets of tuple tuple small tuple put two elements, a first value is stored in the database, the second It is to demonstrate the value of the front end.
Definition field (Gender) time, which can be selected by the representative choices of the field values,

如gender = models.CharField(max_length = 32, choices = gender_choice)。

When the value of the object .get_xxx_display () such as: user_obj.get_gender_display () 

choices parameters of 
the user's gender 
education 
Marital status 
job status 
customers source 
when you list your data can be completely 
you can consider using the parameter choices 


class Userinfo (models.Model): 
    username = models.CharField (max_length = 32) 
    gender_choices = ( 
        ( 1, 'M'), 
        (2, 'F'), 
        (3, 'other') 
    ) 
    Gender = models.IntegerField (choices = gender_choices) 
    # number, or store the field and may be a number other than the matching relation 
    record_choices = (( 'the checked', "checked"), 
                      ( "vacate", "leave"), 
                      ( "lATE", "late"), 
                      ( 'NOSHOW', "absence"), 
                      ( 'leave_early', "早退"),
                      )

    record = models.CharField ( "class record", = record_choices choices, default = "the checked", MAX_LENGTH = 64) 


USER_OBJ = models.Userinfo.objects.get (= PK. 1) 
Print (user_obj.username) 
Print (user_obj.gender ) 
# for choices parameter field value when get_xxx_display () 
Print (user_obj.get_gender_display ()) 
# acquired for data get_xxx_display not comment information () or the number itself 
user_obj = models.Userinfo.objects.get (pk = 4 ) 
Print (user_obj.gender) 
Print (user_obj.get_gender_display ())

　　

 

Database query optimization

defer extension, extension to take in brackets

only only, just take the brackets

 

 

 

select_related与prefetch_related

select_related (even operating table)

select_related will be even table operation, connect all the tables have relationships, and query only once. All fields to obtain information related to the result table encapsulated object. After only need to check the properties required by a period character on the line, no database query.

the select_related () parentheses could send foreign key fields, and not many to many foreign key .

select_related (1__ foreign key foreign key foreign key 2__ 3), connected by a foreign key table indefinitely.

 

prefetch_related (subquery) prefetch data prefetching

By way of internal subqueries (because the database statement performs two (multiple) times, as a result of the first second conditional statement), the outer key and the associated tables all fields in this table to keep the object results , the value only need to value through a period character.

Foreign keys plurality of pairs only through this way .

 

 

 

https://www.cnblogs.com/tuifeideyouran/p/4232028.html

 

django orm open transaction operations

Four characteristics Affairs acid

Atomicity (Atomicity), consistency (Consistency), isolation (Isolation), persistent (Durability Rev)

Open database transactions

the try: 
    SQL statement block 
except exception:　
    ROLLBACK; 
the else:   
    the commit;

　　

Three database design paradigm:

First paradigm (NF1): Each field must be atomic, no longer split 

second paradigm (NF2): each of the primary key fields have correlation. 

Third paradigm (NF3): Each field is directly related to the primary key, rather than indirectly related

　

django orm in turn transactional operations

Import Transaction django.db from 
with transaction.atomic (): 
    # ORM statements executed with the same code block belonging to a transaction 
    pass

　Other matters related configuration needs to be supplemented

 

MTV and MVC model

MTV

django known as MTV frame
M: Models
T: Templates
V: views

MVC

M: Models
V: views
C: Controller Controller (distributed routing the urls.py)
Essence: MTV also essentially MVC

 

 

 

thing