I have a PlaceType model related to the Place model via a M2M field. It functions as necessary where I do place.type.create(name='foo') but it ends up having duplicate values for name=foo. That leads to a large amount of duplicate data. I would like the PlaceType table to be unique in the name field. I tried to add unique=True but it throws an error when inserting. I would essentially want to do a get_or_create on the type but in an elegant way.
class Place
...
class PlaceType
name = models.CharField(max_length=40,unique=True)
place = models.ManyToManyField(Place,related_name="types")
Thank you!
Related
i was join two table and i want to get field in joined model class
school_name_list = Students.objects.select_related('School').values('school_name')
but this code raise
django.core.exceptions.FieldError: Cannot resolve keyword 'school_name' into field``
how can i solve it?
If School is foreign key attribute in Students model, then use School__school_name in values.
students = Students.objects.values('School__school_name')
If you are accessing only a particular field like School__school_name only (using values), there is no need to add select_related because fields that are in values will be fetched.
Let us say I have a model which contains related (foreign key) fields. Likewise, those Foreign Key fields may refer to models which may or may not contain related fields. Note that relational fields in Django may be one-to-one, many-to-one, or many-to-many.
Now, given an instance of a model, I want to recursively and dynamically get all instances of the models related to it, either directly or indirectly down the line. Conceptually, i want to perform a traversal of the related objects and return them.
Example:
class Model1{
rfield1 = models.ForeignKey("Model2")
rfield2 = models.ManyToManyField("Model3")
normalfield1 = models.Charfield(max_length=50)
}
class Model2{
sfield = models.ForeignKey("Model3")
normalfield = models.CharField(max_length=50)
}
class Model3{
normalfield = models.CharField(max_length=50)
}
Let's say, I have an instance of model Model1 model1, and I want to get objects directly related to it i.e. all Model2 and Model3 objects, and also those which are indirectly related i.e. all Model3 objects related to the Model2 objects retrieved previously. I also want to consider the case of a One-to-One field where the related field is defined on the OTHER MODEL.
Also, note that it might not be the case that I know the model of an instance I'm currently working on. Let's say in the previous example, I may not now that model1 is an instance of Model1 model. So I want to perform all these dynamically.
In order to this, I think I need a way to get all related fields of an object.
How to get all the related fields of an object?
And how should I use them to get the actual related objects?
Or is there a way to better to do this? Thank you very much!
UPDATE:
I already know how to perform 1, and 2 basically follows directly from 1. :) Update later.
If you have model1 getting all it's many to many field names (etc) is easy since this is well know and these are all stored in the meta's 'local_many_to_many' list:
[field.name for field in model1._meta.local_many_to_many]
The foreign keys are a bit more tricky since they are stored with all other fields in the meta's 'local_fields' list. Hence we need to make sure that it has a relation of sorts. This can be done as follows:
[field.name for field in model1._meta.local_fields if field.rel]
This method has requires no knowledge of your models. Also further interrogation can be done on the field object if the name is not enough.
I'm having some trouble understanding many-to-many fields in Django.
When I create a many-to-many field, ex:
class GlobalPart (Models.model):
...
category_id=models.ManyToManyField(Category, related_name = 'globalpart')
...
and
class Category (Model.model):
...
category = models.CharField(max_length=250)
...
I notice that it created a new table called appname_globalpart_category_id in addition to the appname_globalpart table for the GlobalPart model.
What I'm wondering is, how should the field types in that table be defined. I would think that
there should be at least one foreign key there to relate the fields. But instead there is the primary key for the table, and the other fields are integers (globalpart_id and category_id).
So my question is -- is that normal? Or did I somehow define the many-to-many field incorrectly? And my next question is how would I get all the category_ids associated to a particular GlobalPart?
(1) short answer: Yes this is normal.
Long answer: ManyToMany table will need a foreign key to both Category and GlobalPart tables. Strictly speaking those two foreign keys should be sufficient. The extra pk that you see in there is just for django magic. You can really get away with only those two foreign keys in that table if you manually define the many-to-many table yourself. However if you let django do it for you (by using ManyToManyField) you get this extra pk
(2) I suggest changing your model fields category_id to categories:
class GlobalPart (Models.model):
categories=models.ManyToManyField(Category, related_name = 'globalpart')
This is because, ManyToManyFields refers well to "many" items. This field does not refer to "one" category_id, it refers to all related categories. So when naming it would be natural to name it accordingly.
As for accessing all categories you can do it by accessing the "categories" property. Say if your object instance named global_part, you can access categories like this:
categories = global_part.categories.all()
Instead of all(), you can use filter() or exclude() the same way you use it when querying models.
Here is a link to related django docs
What do you think a foreign key is? It's a field containing values that equate to IDs - usually primary keys - in the "foreign" table. If the other table has integer keys, as most Django tables do, then the foreign key field will be of type integer as well.
Additionally, Django creates constraints so that the database will enforce that the IDs do actually reference valid values in the foreign table. Depending on your database, these might or might not be displayed as part of the field definition.
I have DB that should have one field with type Many-To-Many, but it is not and I can't change this.
For example I have a list of students and a list of subjects. Subject should be many-to-many field in students table, but as i said it is not. Students table doesn't have this field at all. But there is still another table students-subjects that contains subject_id-student_id items.
How can I embed in student form some kind of subject field that could change students-subjects data when I save a student in DB? The problem is that I can't change the DB structure so need to make it only with help of Django.
There is no such thing as a 'Many-to-many field' for a database-table like you might know it of foreign keys. In database representation many-to-many relationships are realized by using an extra table that links the primary keys (usually the ids) of the records you want to set in relation. In your case that is the table students-subjects. Django uses this table when you define a many-to-many relationship in the model. You do not have to change your database structure at all, it will be working perfectly as it is now.
See the documentation: ManyToManyField
You'll have to set the db_table option with the name of your intermediary table (i.e. students-subjects). Then everything should work fine.
EDIT:
Considering your comment, the problem is that Django expects a certain naming convention (i.e. MODELNAME_id) which isn't provided by your table. Since you say that you cannot change the table itself, you have to try something else.
You have to create an extra Model for your intermediary table (students-subjects) and define the field 'students' as a foreign key to the students model and the field 'subjects' as a foreign key to the subjects model. Then for the many-to-many field you specifiy the option 'through' with the name of your intermediary table. Set the options 'db_column' to let Django know which names you'd like to use for the databse columns. 'db_table' in the meta class is needed to specify your database table name.
You get something like:
class StudentsSubjects(models.Model):
student = models.ForeignKey(Student, db_column='student')
subject = models.ForeignKey(Subject, db_column='subject')
class Meta:
db_table = 'students-subjects'
class Student(models.Model):
...
subjects = models.ManyToManyField(Subject, through='StudentsSubjects')
...
class Subject(models.Model):
...
I hope that will help you.
For more detail see: Extra fields on many-to-many relationship.
I wanted a Django model with 2 foreign keys from the same table. It's an event table which has 2 columns for employees: the 'actor' and the 'receiver'. But I get this error:
Error: One or more models did not validate: tasks.task: Intermediary
model TaskEvent has more than one foreign key to Employee, which is
ambiguous and is not permitted.
Is there a better way to model this?
I think I'm going to add a TaskEvent_to_Employee table. There will be two records in it, one for each of the two employees related to each TaskEvent. Does anyone know an easier workaround?
I haven't done this yet, but I used inspectdb to generate the models.py file from an existing DB that does exactly that - this is what inspectdb threw back, so it should work:
creator = models.ForeignKey(Users, null=True, related_name='creator')
assignee = models.ForeignKey(Users, null=True, related_name='assignee')
Hope that works for you - if it doesn't I am going to have a problem too.
I think what you're looking for is the related_name property on ForeignKeyFields. This will allow you to reference the same table, but give django special names for the relationship.
More Info:
https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.ForeignKey.related_name
https://docs.djangoproject.com/en/dev/topics/db/queries/#backwards-related-objects
https://docs.djangoproject.com/en/dev/topics/db/examples/many_to_one/
From the error message, it sounds like you're trying to put two foreign keys to the same object on an intermediary table used via the through argument to ManyToManyField, the documentation for which states:
When you set up the intermediary
model, you explicitly specify foreign
keys to the models that are involved
in the ManyToMany relation. This
explicit declaration defines how the
two models are related.
There are a few restrictions on the
intermediate model:
Your intermediate model must contain one - and only one - foreign key to
the target model (this would be Person
in our example). If you have more than
one foreign key, a validation error
will be raised.
Your intermediate model must contain one - and only one - foreign key to
the source model (this would be Group
in our example). If you have more than
one foreign key, a validation error
will be raised.
Using related_name was my solution:
class Sample(models.model):
...
class Mymodel(models.model):
example1 = models.ForeignKey(Sample, related_name='sample1')
example2 = models.ForeignKey(Sample, related_name='sample2')
The fact that two columns are part of one table implies that the two fields are related, therefor to reference them individually is not ideal. The ForeignKey of your model should be the primary key of the table you are referencing:
event = models.ForeignKey('event')
You would then reference the columns as such:
foo.event.actor
foo.event.receiver
If you wish you could also change the way your class/model references the foreign attributes with properties. In your class you would do the following:
#property
def actor(self):
return self.event.actor
#property
def receiver(self):
return self.event.receiver
This would allow you to then call foo.actor and foo.receiver but I believe the longer, foo.event.actor would be more pythonic