Foreign keys are almost always “Allow Duplicates,” which would make them unsuitable as Primary Keys. Instead, find a field that uniquely identifies each record in the table, or add a new field (either an auto-incrementing integer or a GUID) to act as the primary key. The only exception to this are tables with a one-to-one … Read more
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 … Read more
I’m guessing that Clients.Case_Number and/or Staff.Emp_ID are not exactly the same data type as Clients_has_Staff.Clients_Case_Number and Clients_has_Staff.Staff_Emp_ID. Perhaps the columns in the parent tables are INT UNSIGNED? They need to be exactly the same data type in both tables.
Because these two lines … … don’t take a parameter in the constructor, I guess that you create a context within the classes. When you load the city1… …you attach the city1 to the context in CityService. Later you add a city1 as a reference to the new Employee e1 and add e1 including this reference to city1 to the context in EmployeeService. As a result you have city1 attached to … Read more
I’ve set up two tables: After I insert data into A, it looks like this: After I insert data into B, it looks like this: I then execute the following statement: I get the following: “Error: foreign key constraint failed” I then restart sqlite3 and try again but this time I enter this first: it … Read more
It’s because the name column on the bar table does not have the UNIQUE constraint. So imagine you have 2 rows on the bar table that contain the name ‘ams’ and you insert a row on baz with ‘ams’ on bar_fk, which row on bar would it be referring since there are two rows matching?
It occurred because you tried to create a foreign key from tblDomare.PersNR to tblBana.BanNR but/and the values in tblDomare.PersNR didn’t match with any of the values in tblBana.BanNR. You cannot create a relation which violates referential integrity.
It’s true that if your primary key is just a identity value auto incremented, you would have no real use for ON UPDATE CASCADE. However, let’s say that your primary key is a 10 digit UPC bar code and because of expansion, you need to change it to a 13-digit UPC bar code. In that … Read more
I found the generated set of drop statements useful, and recommend these tweaks: Limit the generated drops to your database like this: Note 1: This does not execute the DROP statements, it just gives you a list of them. You will need to cut and paste the output into your SQL engine to execute them. Note … Read more
Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). That … Read more