I have two models:
class BusinessCard(models.Model):
name = models.CharField(_("name"),null=True,max_length=50)
class Contacts(models.Model):
businesscard_id = models.OneToOneField(BusinessCard,null=True,blank=True,related_name='contact_detail',db_column="businesscard_id")
bcard_json_data = JsonField(null=True)
I just want access contacts model data using business card model:
target_bacard=BusinessCard.objects.filter(id=target_bacard_id).select_related()
When we access the target_bacard.contact_detail it gives key errors.
How can I get the contacts data using target_bacard queryset.
use get() instead of filter() like:
target_bacard = BusinessCard.objects.get(id=target_bacard_id)
target_bacard.contact_detail
If you want to access the Contacts instance that is in the 1-to-1 relationship with a BusinessCard instance bacard, use the related name you specified in Contacts:
contact = bacard.contact_detail
Also, you have some misleading names: Contacts should rather be Contact since an instance of this model represents only one contact. And its field businesscard_id would better be named businesscard (note that the table column will be called businesscard_id at the database level automatically in that case and store the id of the related businesssscard) because in the ORM you get a BusinessCard model instance when you access it, and not just its id.
You have not passed related model (field) argument to select_related()
target_bacard=BusinessCard.objects.filter(id=target_bacard_id).select_related('contact_detail')
Assuming id of BusinessCard is unique, you may want to use ...objects.get(id=target_bacard_id) inplace of ...objects.filter(id=target_bacard_id). Anyway select_related() will work on both ways.
select_related() is used for saving database query.
here is the documentation
Related
What is the right way to lookup a table and use its last value as a value in a new model instance? Something like this:
class MyClass(models.Model):
id = models.AutoField(primary_key=True)
obj = MyClass.objects.latest('id')
my_field = models.IntegerField(default=(obj+1))
I need a db column, which keeps track of the primary_key, but is independent of it and can also be modified. Also I need to use its value as default when creating new instances of the Model.
you can use custom constructor as described in the docs:
https://docs.djangoproject.com/en/2.2/ref/models/instances/
you will need to define the obj field either as integer(to store the id of the previous record) or as a foreign key(if you want to reference the previous db record). In the second case you will need to pass the name of the model to the ForeignKeyField constructor as string('MyClass') and not directly(MyClass).
I am using Simple-Salesforce to query records via .query_all, but when I include a recently created custom field, I receive the No such column error.
An example of the query that creates the error is below, with Problem_Field__c as a stand-in for my field.
s.query_all('SELECT ID, Name, Problem_Field__c FROM Custom_Object___c')
I have already reviewed the field-level security of this field and do have access to it.
As additional information, my login to the sandbox in which I am using this custom field is below:
s = simple_salesforce.Salesforce(username='myUsername.TestDomain',
password='myPassword',
organizationId='mySandboxOrgId',
security_token='',
domain='test')
The problem field is a lookup field to the Contact object.
A lookup is a relationship between two objects. When you use a relationship in a query and the query is navigating the relationship in a child-to-parent direction (Contact = parent, your custom object = child), you must use the relationship names. Custom relationships are named with an __r rather than __c suffix (docs). The relationship name is typically the same as the API name of the lookup on Lookup definition screen but with suffix replaced. Your query should be
s.query_all('SELECT ID, Name, MyRelationship__r.Some_Contact_Field FROM Custom_Object___c')
To know the relationship name for sure, you can use take a look at the object schema.
I have the following models:
class Company(ndb.Model):
name = ndb.StringProperty(indexed=False)
# some other fields
class User(polymodel.PolyModel):
company = ndb.KeyProperty(kind=Company)
# some other fields
class Object(ndb.Model):
user = ndb.KeyProperty(kind=User)
# some other fields
Now I have a user and I want to query Objects that are associated with other Users in the same company like this:
Object.query(Object.user.company == user.company)
Of course, this doesn't work, since Object.user is a key and I cannot access anything beyond that.
Is there any way to do it? I only need the company key, I was thinking on a ComputedProperty but I'm not sure if it's the best solution. Also, it would be better to query based on any field in company.
You need to denormalize and store redundant information, as the datastore doesn't support joins.
For instance given your models above, a user can only be a member of one company, if you really need to search all objects whose user is a member of a particular company then store the company key in the Object.
Use a computed property if that works best for you.
Alternately use a factory that always takes the User as a argument and construct Object that way.
How does one update - in bulk - a many to many field in the queryset of a Django data model?
For instance, I have a data model called Photo, and another called PhotoStream. In Photo, I have which_stream = models.ManyToManyField(PhotoStream).
I extracted a queryset of Photos called childhood_photos, and I need to add a new PhotoStream object in the many-to-many fields of all objects within this queryset. Let's call this PhotoStream object for_classmates.
I try childhood_photos.update(which_stream.add(for_classmates)), but it gives me an error: global name 'which_stream' is not defined.
How can I do this operation?
You can access the through model of your m2n relationship via the field's .through attribute (see the documentation). That will allow you to bulk create the necessary through model instances:
through_model = Photo.which_stream.through # gives you access to auto-created through model
# print through_model
# <class 'app_label.models.Photo_which_stream'> # or sth. similar
through_model.objects.bulk_create([
through_model(photo_id=pk, photostream_id=for_classmates.pk)
for pk in childhood_photos.values_list('pk', flat=True)
])
This question has been asked before, but the answers there do not solve my problem.
I am using a legacy database, nothing can be changed
Here are my django models, with all but the relevant fields stripped off, obviously class meta has Managed=False in my actual code:
class AppCosts(models.Model):
id = models.CharField(primary_key=True)
cost = models.DecimalField()
class AppDefs(models.Model):
id = models.CharField(primary_key=True)
data = models.TextField()
appcost = models.OneToOneField(AppCosts, db_column='id')
class JobHistory(models.Model):
job_name = models.CharField(primary_key=True)
job_application = models.CharField()
appcost = models.OneToOneField(AppCosts, to_field='id', db_column='job_application')
app = models.OneToOneField(AppDefs, to_field='id', db_column='job_application')
The OneToOne fields work fine for querying, and I get the correct result using select_related()
But when I create a new record for the JobHistory table, when I call save(), I get:
DatabaseError: (1110, "Column 'job_application' specified twice")
I am using django 1.4 and I do not quite get how this OneToOneField works. I can't find any example where primary keys are named differently and has this particular semantics
I need the django model that would let me do this SQL:
select job_history.job_name, job_history.job_application, app_costs.cost from job_history, app_costs where job_history.job_application = app_costs.id;
You have defined appcost and app to have the same underlying database column, job_application, which is also the name of another existing field: so three fields share the same column. That makes no sense at all.
OneToOneFields are just foreign keys constrained to a single value on both ends. If you have foreign keys from JobHistory to AppCost and AppDef, then presumably you have actual columns in your database that contain those foreign keys. Those are the values you should be using for db_field for those fields, not "job_application".
Edit I'm glad you said you didn't design this schema, because it is pretty horrible: you won't have any foreign key constraints, for example, which makes referential integrity impossible. But never mind, we can actually achieve what you want, more or less.
There are various issues with that you have, but the main one is that you don't need the separate "job_application" field at all. That is, as I said earlier, the foreign key, so let it be that. Also note it should be an actual foreign key field, not a one-to-one, since there are many histories to one app.
One constraint that we can't achieve easily in Django is to have the same field acting as FK for two tables. But that doesn't really matter, since we can get to AppCosts via AppDefs.
So the models could just look like this:
class AppCosts(models.Model):
app = models.OneToOneField('AppDefs', primary_key=True, db_field='id')
cost = models.DecimalField()
class AppDefs(models.Model):
id = models.CharField(primary_key=True)
data = models.TextField()
class JobHistory(models.Model):
job_name = models.CharField(primary_key=True)
app = models.ForeignKey(AppDefs, db_column='job_application')
Note that I've moved the one-to-one between Costs and Defs onto AppCosts, since it seems to make sense to have the canonical ID in Defs.
Now, given a JobHistory instance, you can do history.app to get the app instance, history.app.cost to get the app cost, and use the history.app_id to get the underlying app ID from the job_application column.
If you wanted to reproduce that SQL output more exactly, something like this would now work:
JobHistory.objects.values_list('job_name', 'app_id', 'app__appcosts__cost')