python - working - django list field
differentiate null=True, blank=True in django (8)
When we add a database field in django we generally write
models.CharField(max_length=100, null=True, blank=True). The same is done with
DecimalField etc. What is the basic difference in having
in respect to different (
DateTimeField) fields. What are the advantages/disadvantages of using 1/2/3?
NOT NULL) on the column in your DB. Blank values for Django field types such as
ForeignKey will be stored as
NULL in the DB.
blank=True determines whether the field will be required in forms. This includes the admin and your own custom forms. If
blank=True then the field will not be required, whereas if it's
False the field cannot be blank.
The combo of the two is so frequent because typically if you're going to allow a field to be blank in your form, you're going to also need your database to allow
NULL values for that field. The exception is
TextFields, which in Django are never saved as
NULL. Blank values are stored in the DB as an empty string (
A few examples:
models.DateTimeField(blank=True) # raises IntegrityError if blank models.DateTimeField(null=True) # NULL allowed, but must be filled out in a form
Obviously those two options don't make logical sense to use (though, there might be a use case for
null=True, blank=False if you want a field to always be required in forms, but optional when dealing with an object through something like the shell.)
models.CharField(blank=True) # No problem, blank is stored as '' models.CharField(null=True) # NULL allowed, but will never be set as NULL
TEXT types are never saved as
NULL by Django, so
null=True is unnecessary. However, you can manually set one of these fields to
None to force set it as
NULL. If you have a scenario where that might be necessary, you should still include
As said in Django Model Field reference: Link
The following arguments are available to all field types. All are optional.
True, Django will store empty values as
NULL in the database. Default is
null on string-based fields such as
TextField because empty string values will always be stored as empty strings, not as
NULL. If a string-based field has
null=True, that means it has two possible values for "no data":
NULL, and the empty string. In most cases, it’s redundant to have two possible values for "no data"; the Django convention is to use the empty string, not
For both string-based and non-string-based fields, you will also need to set
blank=True if you wish to permit empty values in forms, as the
null parameter only affects database storage (see
When using the Oracle database backend, the value NULL will be stored to denote the empty string regardless of this attribute
True, the field is allowed to be blank. Default is
Note that this is different than
null is purely database-related, whereas
blank is validation-related. If a field has
blank=True, form validation will allow entry of an empty value. If a field has
blank=False, the field will be required.
Here, is the main difference of
The default value of both
blank is False. Both of these values work at field level i.e., whether we want to keep a field
null=True will set the field’s value to
NULL i.e., no data. It is basically for the databases column value.
date = models.DateTimeField(null=True)
blank=True determines whether the field will be required in forms. This includes the admin and your own custom forms.
title = models.CharField(blank=True) // title can be kept blank.
In the database
("") will be stored.
blank=True This means that the field is optional in all circumstances.
epic = models.ForeignKey(null=True, blank=True) // The exception is CharFields() and TextFields(), which in Django are never saved as NULL. Blank values a
I think you may be interested in Save empty, nullable CharField's as null rather than as an empty string. There are many discussions about this, and a very practical problem you may encounter (eg. you want to add a openid url for each user which can be null and should be unique).
There's one point where
null=True would be necessary even on a
TextField and that is when the database has the
unique flag set for the column.
In other words, if you've a unique Char/TextField in Django, you'll need to use this:
models.CharField(blank=True, null=True, unique=True)
For non-unique CharField or TextField, you'll be better off skipping the
null=True otherwise some fields will get set as NULL while others as "" , and you'll have to check the field value for NULL everytime.
This is how the ORM maps
null fields for Django 1.8
class Test(models.Model): charNull = models.CharField(max_length=10, null=True) charBlank = models.CharField(max_length=10, blank=True) charNullBlank = models.CharField(max_length=10, null=True, blank=True) intNull = models.IntegerField(null=True) intBlank = models.IntegerField(blank=True) intNullBlank = models.IntegerField(null=True, blank=True) dateNull = models.DateTimeField(null=True) dateBlank = models.DateTimeField(blank=True) dateNullBlank = models.DateTimeField(null=True, blank=True)
The database fields created for PostgreSQL 9.4 are :
CREATE TABLE Test ( id serial NOT NULL, "charNull" character varying(10), "charBlank" character varying(10) NOT NULL, "charNullBlank" character varying(10), "intNull" integer, "intBlank" integer NOT NULL, "intNullBlank" integer, "dateNull" timestamp with time zone, "dateBlank" timestamp with time zone NOT NULL, "dateNullBlank" timestamp with time zone, CONSTRAINT Test_pkey PRIMARY KEY (id) )
The database fields created for MySQL 5.6 are :
CREATE TABLE Test ( `id` INT(11) NOT NULL AUTO_INCREMENT, `charNull` VARCHAR(10) NULL DEFAULT NULL, `charBlank` VARCHAR(10) NOT NULL, `charNullBlank` VARCHAR(10) NULL DEFAULT NULL, `intNull` INT(11) NULL DEFAULT NULL, `intBlank` INT(11) NOT NULL, `intNullBlank` INT(11) NULL DEFAULT NULL, `dateNull` DATETIME NULL DEFAULT NULL, `dateBlank` DATETIME NOT NULL, `dateNullBlank` DATETIME NULL DEFAULT NULL )
When we save anything in Django admin two steps validation happens, on Django level and on Database level. We can't save text in a number field.
Database has data type NULL, it's nothing. When Django creates columns in the database it specifies that they can't be empty. And if you will try to save NULL you will get the database error.
Also on Django-Admin level, all fields are required by default, you can't save blank field, Django will throw you an error.
So, if you want to save blank field you need to allow it on Django and Database level. blank=True - will allow empty field in admin panel null=True - will allow saving NULL to the database column.