Fields¶
The Field class is used to describe the mapping of Model attributes to database columns. Each field type has a corresponding SQL storage class (i.e. varchar, int), and conversion between python data types and underlying storage is handled transparently.
When creating a Model class, fields are defined as class-level attributes. This should look familiar to users of the django framework. Here’s an example:
from peewee import * class User(Model): username = CharField() join_date = DateTimeField() about_me = TextField()
There is one special type of field, ForeignKeyField , which allows you to expose foreign-key relationships between models in an intuitive way:
class Message(Model): user = ForeignKeyField(User, related_name='messages') body = TextField() send_date = DateTimeField()
This allows you to write code like the following:
>>> print some_message.user.username Some User >>> for message in some_user.messages: . print message.body some message another message yet another message
Field types table¶
Parameters accepted by all field types and their default values:
- null = False – boolean indicating whether null values are allowed to be stored
- db_index = False – boolean indicating whether to create an index on this column
- unique = False – boolean indicating whether to create a unique index on this column
- verbose_name = None – string representing the “user-friendly” name of this field
- help_text = None – string representing any helpful text for this field
- db_column = None – string representing the underlying column to use if different, useful for legacy databases
Some fields take special parameters. ¶
| Field type | Special Parameters |
|---|---|
| CharField | max_length |
| DateTimeField | formats |
| DateField | formats |
| TimeField | formats |
| DecimalField | max_digits , decimal_places , auto_round , always_float |
| ForeignKeyField | to , related_name , cascade , extra |
Self-referential Foreign Keys¶
Since the class is not available at the time the field is declared, when creating a self-referential foreign key pass in ‘self’ as the “to” relation:
class Category(Model): name = CharField() parent = ForeignKeyField('self', related_name='children', null=True)
Implementing Many to Many¶
Peewee does not provide a “field” for many to many relationships the way that django does – this is because the “field” really is hiding an intermediary table. To implement many-to-many with peewee, you will therefore create the intermediary table yourself and query through it:
class Student(Model): name = CharField() class Course(Model): name = CharField() class StudentCourse(Model): student = ForeignKeyField(Student) course = ForeignKeyField(Course)
To query, let’s say we want to find students who are enrolled in math class:
for student in Student.select().join(StudentCourse).join(Course).where(name='math'): print student.name
You could also express this as:
for student in Student.filter(studentcourse_set__course__name='math'): print student.name
To query what classes a given student is enrolled in:
for course in Course.select().join(StudentCourse).join(Student).where(name='da vinci'): print course.name # or, similarly for course in Course.filter(studentcourse_set__student__name='da vinci'): print course.name
Field class API¶
The base class from which all other field types extend.
- null – this column can accept None or NULL values
- db_index – create an index for this column when creating the table
- unique – create a unique index for this column when creating the table
- verbose_name – specify a “verbose name” for this field, useful for metadata purposes
- help_text – specify some instruction text for the usage/meaning of this field
- lookup_type – a peewee lookup type, such as ‘eq’ or ‘contains’
- value – a python data type
data type converted for use when querying
Simple hook for Field classes to indicate when the Model class the field exists on has been created.
Stores: small strings (0-255 bytes)
Stores: arbitrarily large strings
Stores: python datetime.datetime instances
Accepts a special parameter formats , which contains a list of formats the datetime can be encoded with. The default behavior is:
'%Y-%m-%d %H:%M:%S.%f' # year-month-day hour-minute-second.microsecond '%Y-%m-%d %H:%M:%S' # year-month-day hour-minute-second '%Y-%m-%d' # year-month-day
If the incoming value does not match a format, it will be returned as-is
Stores: python datetime.date instances
Accepts a special parameter formats , which contains a list of formats the date can be encoded with. The default behavior is:
'%Y-%m-%d' # year-month-day '%Y-%m-%d %H:%M:%S' # year-month-day hour-minute-second '%Y-%m-%d %H:%M:%S.%f' # year-month-day hour-minute-second.microsecond
If the incoming value does not match a format, it will be returned as-is
Stores: python datetime.time instances
Accepts a special parameter formats , which contains a list of formats the time can be encoded with. The default behavior is:
'%H:%M:%S.%f' # hour:minute:second.microsecond '%H:%M:%S' # hour:minute:second '%H:%M' # hour:minute '%Y-%m-%d %H:%M:%S.%f' # year-month-day hour-minute-second.microsecond '%Y-%m-%d %H:%M:%S' # year-month-day hour-minute-second
If the incoming value does not match a format, it will be returned as-is
Stores: floating-point numbers
It’s default behavior is to return decimal.Decimal Python object. This is can store really large numbers so Python does not support seamless conversion from Decimal to float.
If the only reason for you to use Decimal at database is to store amount where it always has exactly two decimal places and you happen then it might be easier for you to turn on auto_round and always_float flags.
The former will make sure that if the value has higher precision than the number of decimal_places then it round it to that value before send that to database. This will save from some unexpected “Data truncation” warnings from MySQL backend (.. _ref: http://bit.ly/bWr1mn).
The latter will make sure that in Python code you always get the value as float instead of Decimal . This way you can easily mix other float numbers without tracking their types. Also float can be faster than Decimal .
Stores: auto-incrementing integer fields suitable for use as primary key
Stores: relationship to another model
- to – related Model class or the string ‘self’ if declaring a self-referential foreign key
- related_name – attribute to expose on related model
class Blog(Model): name = CharField() class Entry(Model): blog = ForeignKeyField(Blog, related_name='entries') title = CharField() content = TextField() # "blog" attribute >>> some_entry.blog Blog: My Awesome Blog> # "entries" related name attribute >>> for entry in my_awesome_blog.entries: . print entry.title Some entry Another entry Yet another entry