Is there Way to get Foreign key in ArrayResult ? I have entity with ManyToOne reference to self. I want cache results, but If try cache ResultObject, i get error about serializing and __sleep. So, i try cache arrayResult, but foreign key don't show in array.
Thank for any help.
Related
I have Django project with DigestIssue model among others and there were Django auto created primary key field id and my field number. I wanted to get rid of duplication and set number as PK because number is unique and with same values as id. But I have foreign keys referencing this model.
I doubt that they will migrate automatically after such operation. I tried, hoping for such automigration and got constraint "idx_16528_sqlite_autoindex_gatherer_digestissue_1" of relation "gatherer_digestissue" does not exist error ("sqlite" in constraint name is historical thing, I switched to PostgreSQL a time go). I tried more complicated way, following https://blog.hexack.fr/en/change-the-primary-key-of-a-django-model.html but got same error on PK switching step.
So the question is - how to replace in Django old primary key with new one with same values and referenced by other models?
I'm adapting a really big query (MySQL to SQL) that creates lots of tables and relationships. The problem is that it always check if a table exists prior to adding a foreign key referencing to that table.
So I have to reorder the queries to avoid this problem, and my question is if there's a instruction that can turn that check off, so it'll create the tables and add references without stopping the query with every error it encounters.
I'm working with SQL in an Azure DB.
Thank you.
Easiest way is to create all tables first and then add constraints using ALTER TABLE.
For example:
CREATE TABLE a(id INT PRIMARY KEY IDENTITY(1,1), b_id INT, c CHAR(10));
CREATE TABLE b(id INT PRIMARY KEY IDENTITY(1,1), z INT);
ALTER TABLE a ADD CONSTRAINT FK_a_b_id_b FOREIGN KEY (b_id) REFERENCES b(id);
Rextester Demo
How does select_related work with a model which has multiple foreign keys? Does it just choose the first one?
class Model:
fkey1, fkey2, fkey3...
The documentation doesn't say anything about this, at least not in where the method is specified.
NOTE: Maybe I'm the only one who will get confused. I guess select_related is just a performance booster (I can see that) but I had the wrong idea that it was something else.
You can use select_related in a chain as following
Comment.objects.select_related('user').select_related('article').all()
If your model has multiple foreign keys you can:
Call .select_related(), that will “follow” all non-null foreign-key relationships
Call .select_related('foreign_key1', 'foreign_key2', ...), that will “follow” only the foreign-key provided as arguments.
Note that "to follow a FK relationship" means selecting additional related-object data when the query is executed (by performing a SQL join). This will make the main query heavier but can be used to avoid N + 1 queries problem.
According to select_related documentation, the first method (without arguments) is not recommended as "it is likely to make the underlying query more complex, and return more data, than is actually needed."
If your model has "nested" foreign keys with other models (i.e. Book <>-- Author <>-- Hometown) you can also use select_related as follow:
Call Book.select_related('author__hometown'), that will “follow” the author's foreign-key (in Book model) and the hometown's foreign-key (in Author model).
If your model has many-to-many or many-to-one relations you would like to retrieve from the database, you should take a look at prefetch_related.
On the contrary, the documentation is very clear on the matter. It says that by default all ForeignKeys are followed, but you can give the method a list of fields and it will only follow those relationships.
You can pass foreign keys and even nested foreign keys to the select_related method eg select_related('book__author', 'publisher',) you can add as many foreign keys as you want, if you call select_related() without any argument then it will follow all the foreign key relationship which is not recommended at all because you'll be complicating the query by fetching data you don't need. Finally, from Django documentation "Chaining select_related calls works in a similar way to other methods - that is that select_related('foo', 'bar') is equivalent to select_related('foo').select_related('bar')
Simply put, i want to create a structure that has a component MAKTX, and to have a foreign key relation with MAKT-MAKTX.
More generally i want to have a foreign key check for a field that's not part of a primary key.
I see the button "Non-key-fields/candidates", but i don't really know how to use it.
Also, i don't want to use the "key fields of a text table" relation... but i don't know if that's relevant.
Is this even a good thing that i'm trying to do? I don't see any reason why it shouldn't be possible, but you might object.
[EDIT]: I have to mention that I don't really know what I'm doing. I really just want to fill a table i created with values from another, and to make sure that those values (namely MAKTX - kind ofvalues) in my table are always values from MAKT. Suppose i do the initial filling with a SELECT statement, i want the consistency to work even if i later insert new entries manually.
So I don't know whether this makes sense or not, it just sounds to me like a good idea to have the system perform this check automatically, if possible.
Main condition for creating foreign key relation is that the field should be a primary key in your reference table. While in the table you are creating foreign key its not necessary that the field is a primary key or not. The main reason for this is that foreign key cant be null.
Refer to below link for step by step process for creating foreign key relation in abap.
http://learnabaponline.blogspot.in/2013/04/how-to-create-table-in-abap.html
First off, I agree with vwegerts's comments, what you're trying to do doesn't seem to make any sense.
Perhaps this would make more sense: create your own table without the MAKTX field. Then create a database view, joining your table and the MAKT table (and set a default language in the selection conditions if you want to). This way you'll have the descriptions joined with your data, without duplicating the actual data (which is what it looks like you're trying to do).
I have a legacy database with a table storing a many-to-many relationship, but without a single primary key column. Is there any way to convince Django to use it anyway?
Schematically:
Product 1<---->* Labeling *<---->1 Label
The Labeling table uses (product_id,label_id) as a compound primary key, and I don't see any way to inform Django about this. (Just using through gives me Unknown column 'labeling.id' in 'field list'.)
Do I need to fall back to custom SQL? Or am I missing something?
hope this helps you,
http://docs.djangoproject.com/en/dev/ref/models/options/#unique-together
http://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.Field.db_index
If you add a unique_together to the model for the many-to-many table, Django will use those columns instead of expecting a primary key called id.