What is the most efficient way to store a list in the Django models

Question

Currently I have a lot of python objects in my code similar to the following   class MyClass      def   init   self  name  friends         self myName   name       self myFriends    str x  for x in friends    Now I want to turn this into a Django model  where self myName is a string field  and self myFriends is a list of strings   from django db import models  class myDjangoModelClass        myName   models CharField max length 64      myFriends         what goes here    Since the list is such a common data structure in python  I sort of expected there to be a Django model field for it  I know I can use a ManyToMany or OneToMany relationship  but I was hoping to avoid that extra indirection in the code   Edit   I added this related question  which people may find useful

User · Answer

Storing a list of strings in Django model   class Bar models Model       foo   models TextField blank True       def set list self  element           if self foo              self foo   self foo         element         else              self foo   element      def get list self           if self foo              return self foo split              else              None   and you can call it like this   bars   Bar   bars set list  str1   bars set list  str2   list   bars get list   if list is not None      for bar in list          print bar else      print  List is empty

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In case you re using postgres  you can use something like this   class ChessBoard models Model        board   ArrayField          ArrayField              models CharField max length 10  blank True               size 8                     size 8          if you need more details you can read in the link below  https   docs djangoproject com pt-br 1 9 ref contrib postgres fields

User · Answer

Remember that this eventually has to end up in a relational database  So using relations really is the common way to solve this problem  If you absolutely insist on storing a list in the object itself  you could make it for example comma-separated  and store it in a string  and then provide accessor functions that split the string into a list  With that  you will be limited to a maximum number of strings  and you will lose efficient queries

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class Course models Model      name   models CharField max length 256     students   models ManyToManyField Student   class Student models Model      first name   models CharField max length 256     student number   models CharField max length 128       other fields  etc        friends   models ManyToManyField  self

User · Answer

Premature optimization is the root of all evil    With that firmly in mind  let s do this  Once your apps hit a certain point  denormalizing data is very common  Done correctly  it can save numerous expensive database lookups at the cost of a little more housekeeping   To return a list of friend names we ll need to create a custom Django Field class that will return a list when accessed   David Cramer posted a guide to creating a SeperatedValueField on his blog  Here is the code   from django db import models  class SeparatedValuesField models TextField         metaclass     models SubfieldBase      def   init   self   args    kwargs           self token   kwargs pop  token                super SeparatedValuesField  self    init    args    kwargs       def to python self  value           if not value  return         if isinstance value  list               return value         return value split self token       def get db prep value self  value           if not value  return         assert isinstance value  list  or isinstance value  tuple           return self token join  unicode s  for s in value        def value to string self  obj           value   self  get val from obj obj          return self get db prep value value    The logic of this code deals with serializing and deserializing values from the database to Python and vice versa  Now you can easily import and use our custom field in the model class   from django db import models from custom fields import SeparatedValuesField   class Person models Model       name   models CharField max length 64      friends   SeparatedValuesField

User · Answer

Using one-to-many relation  FK from Friend to parent class  will make your app more scalable  as you can trivially extend the Friend object with additional attributes beyond the simple name   And thus this is the best way

User · Answer

You can store virtually any object using a Django Pickle Field  ala this snippet   http   www djangosnippets org snippets 513

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As this is an old question  and Django techniques must have changed significantly since  this answer reflects Django version 1 4  and is most likely applicable for v 1 5   Django by default uses relational databases  you should make use of  em  Map friendships to database relations  foreign key constraints  with the use of ManyToManyField  Doing so allows you to use RelatedManagers for friendlists  which use smart querysets  You can use all available methods such as filter or values list   Using ManyToManyField relations and properties   class MyDjangoClass models Model       name   models CharField          friends   models ManyToManyField  self         property     def friendlist self             Watch for large querysets  it loads everything in memory         return list self friends all      You can access a user s friend list this way   joseph   MyDjangoClass objects get name  Joseph   friends of joseph   joseph friendlist   Note however that these relations are symmetrical  if Joseph is a friend of Bob  then Bob is a friend of Joseph

User · Answer

Would this relationship not be better expressed as a one-to-many foreign key relationship to a Friends table   I understand that myFriends are just strings but I would think that a better design would be to create a Friend model and have MyClass contain a foreign key realtionship to the resulting table

User · Answer

If you are using Django    1 9 with Postgres you can make use of ArrayField advantages      A field for storing lists of data  Most field types can be used  you   simply pass another field instance as the base field  You may also   specify a size  ArrayField can be nested to store multi-dimensional   arrays    It is also possible to nest array fields   from django contrib postgres fields import ArrayField from django db import models  class ChessBoard models Model       board   ArrayField          ArrayField              models CharField max length 10  blank True               size 8                     size 8          As  thane-brimhall mentioned it is also possible to query elements directly  Documentation reference

User · Answer

A simple way to store a list in Django is to just convert it into a JSON string  and then save that as Text in the model  You can then retrieve the list by converting the  JSON  string back into a python list  Here s how   The  list  would be stored in your Django model like so   class MyModel models Model       myList   models TextField null True    JSON-serialized  text  version of your list   In your view controller code   Storing the list in the database   import simplejson as json   this would be just  import json  in Python 2 7 and later          myModel   MyModel   listIWantToStore    1 2 3 4 5  hello   myModel myList   json dumps listIWantToStore  myModel save     Retrieving the list from the database   jsonDec   json decoder JSONDecoder   myPythonList   jsonDec decode myModel myList    Conceptually  here s what s going on    gt  gt  gt  myList    1 2 3 4 5  hello    gt  gt  gt  import simplejson as json  gt  gt  gt  myJsonList   json dumps myList   gt  gt  gt  myJsonList   1  2  3  4  5   hello     gt  gt  gt  myJsonList   class    lt type  str  gt   gt  gt  gt  jsonDec   json decoder JSONDecoder    gt  gt  gt  myPythonList   jsonDec decode myJsonList   gt  gt  gt  myPythonList  1  2  3  4  5  u hello    gt  gt  gt  myPythonList   class    lt type  list  gt

[python] What is the most efficient way to store a list in the Django models?

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