What is related name used for in Django

Question

What is the related name argument useful for on ManyToManyField and ForeignKey fields  For example  given the following code  what is the effect of related name  maps    class Map db Model       members   models ManyToManyField User  related name  maps                                        verbose name    members

User · Answer

The related name parameter is actually an option  If we do not set it  Django automatically creates the other side of the relation for us  In the case of the Map model  Django would have created a map set attribute  allowing access via m map set in your example m being your class instance   The formula Django uses is the name of the model followed by the string  set  The related name parameter thus simply overrides Django   s default rather than providing new behavior

User · Answer

prefetch related use for prefetch data for Many to many and many to one relationship data  select related is to select data from a single value relationship  Both of these are used to fetch data from their relationships from a model  For example  you build a model and a model that has a relationship with other models  When a request comes you will also query for their relationship data and Django has very good mechanisms To access data from their relationship like book author name but when you iterate a list of models for fetching their relationship data Django create each request for every single relationship data  To overcome this we do have  prefetchd related and selected related

User · Answer

The essentials of your question are as follows  Since you have Map and User models and you have defined ManyToManyField in Map model  if you want to get access to members of the Map then you have the option of map instance members all   since you have defined members field  However  say you want to access all maps a user is a part of then what option do you have  By default  Django provided you with user instance modelname set all   and this will translate to the user map set all   in this case  maps is much better than map set  related name provides you an ability to let Django know how you are going to access Map from User model or in general how you can access reverse models which is the whole point in creating ManyToMany fields and using ORM in that sense

User · Answer

To add to existing  answer - related name is a must in case there 2 FKs in the model that point to the same table  For example in case of Bill of material   with author  class BOM models Model        name   models CharField max length 200 null True  blank True      description   models TextField null True  blank True      tomaterial    models ForeignKey Material  related name    tomaterial       frommaterial    models ForeignKey Material  related name    frommaterial       creation time   models DateTimeField auto now add True  blank True      quantity   models DecimalField max digits 19  decimal places 10    So when you will have to access this data  you only can use related name    bom   material tomaterial all   order by  -creation time     It is not working otherwise  at least I was not able to skip the usage of related name in case of 2 FK s to the same table

User · Answer

The related name argument is also useful if you have more complex related class names  For example  if you have a foreign key relationship   class UserMapDataFrame models Model       user   models ForeignKey User     In order to access UserMapDataFrame objects from the related User  the default call would be User usermapdataframe set all    which it is quite difficult to read    Using the related name allows you to specify a simpler or more legible name to get the reverse relation  In this case  if you specify user   models ForeignKey User  related name  map data    the call would then be User map data all

User · Answer

The related name attribute specifies the name of the reverse relation from the User model back to your model   If you don t specify a related name  Django automatically creates one using the name of your model with the suffix  set  for instance User map set all      If you do specify  e g  related name maps on the User model  User map set will still work  but the User maps  syntax is obviously a bit cleaner and less clunky  so for example  if you had a user object current user  you could use current user maps all   to get all instances of your Map model that have a relation to current user    The Django documentation has more details

[python] What is `related_name` used for in Django?

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