I’m working on a feature. I’ve three different car types (Sedan, Hatchback, SUV): I’ve 6 features in total. Out of 6, 4 features are their in every car. The second car category has 5 and the third has all the 6 features. Here where I’m getting stuck: I’ve to send all the 6 featur…
Tag: django-models
Idempotent record creation: is it better to use a unique constraint or check for existence before inserting a record?
I’ve recently been wondering what’s generally the best approach for ensuring that the creation of database records is idempotent. The two approaches I can think of are: Checking whether a record already exists before executing an INSERT Using a unique constraint on the relevant columns to insure t…
Django ORM Left Join onto same table
I have a custom permissions system in a django project that can link any user to any specific model instance to grant permission on that object. It is more complex than this, but boiled down: I want to query for all permissions for any user, that are linked to the same content_object(s) that a particular user…
Django, have model to be related to many instances of itself
I have a model: I want it to have a sub_industries field that will hold all of its sub industries, which should be instances of Industry model. I cannot be ManyToMany because each industry has only one parent industry. Answer You can add a ForeignKey to itself with: You can use NULL/None to express that an in…
How to work around Django’s lack of composite keys in this complicated case?
I can’t seem to figure out how to work around the lack of composite keys in Django for the following case. I’m going to write the schema I’d like using SQLite3 dialect. (The only thing below that might be unfamiliar to someone who knows SQL but not sqlite is sqlite’s “WITHOUT ROW…
Django: How to mangle through relations
I have an instance of ModelA and want to query all instances of ModelC which are related to ModelA through ModelB. ModelA -> all ModelB instances with FK rel_a = ModelA -> rel_c.all() I know how I would do this in SQL but I really do not get how I should unmangle these relations Answer You can query wit…
In Django, how can I get a count of records based on a subquery of a subquery?
A baseball player plays in a game if he makes one or more appearances in that game. So, to get a count of the number of games a player played in, you need to count the games that have an inning that have an appearance by that player. Here are my models: The SQL query to achieve what I want
How can I temporarily disable a foreign key constraint in MySQL?
Is it possible to temporarily disable constraints in MySQL? I have two Django models, each with a foreign key to the other one. Deleting instances of a model returns an error because of the foreign key constraint: Is it possible to temporarily disable constraints and delete anyway? Answer Try DISABLE KEYS or …