Apply foreign key and check constraint to MySQL table - mysql

Question

My question boils down to something like 'Can join table B be subject to a check constraint against a value in table A which is not a foreign key?'. The situation may be further complicated by multiple references to the same Foreign Key. The specific scenario I am struggling with is detailed below with some abbreviated MySQL code.
A MySQL database contains the table 'Disorder' (shown below):
CREATE TABLE `Disorder` (
disorder_name VARCHAR(255),
disorder_type VARCHAR(10),
PRIMARY KEY(`disorder_name`)
)
The disorder type can be either 'syndrome' or 'disease'. Diseases and syndromes can be linked in a many-to-many fashion (e.g. several diseases can be caused the same syndrome and the same disease may cause different syndromes).
I wish to create a join table called 'DiseaseSyndromeLink' showing the relationship between disorders that are 'diseases' and disorders that are 'syndromes'.
CREATE TABLE `DiseaseSyndromeLink` (
`ds_id` int NOT NULL AUTO_INCREMENT,
`disease` VARCHAR(255) NOT NULL,
`syndrome` VARCHAR(255) NOT NULL,
PRIMARY KEY (`ds_id`),
FOREIGN KEY (disease) REFERENCES disorder(disorder_name),
FOREIGN KEY (syndrome) REFERENCES disorder(disorder_name)
)
This table needs constraints that are tricky to design:
The first column is a integer join ID
The second column 'Disease' is a foreign key referencing Disorder.disorder_name. Only disorder_names where Disorder.disorder_type='Disease' should be allowed to be entered here.
The third column 'Syndrome' is a foreign key referencing Disorder.disorder_name. Only disorder_names where Disorder.disorder_type='Syndrome' should be allowed to be entered here.
I feel the syntax should be something like:
CREATE TABLE `DiseaseSyndromeLink` (
`ds_id` int NOT NULL AUTO_INCREMENT,
`disease` VARCHAR(255) NOT NULL,
`syndrome` VARCHAR(255) NOT NULL,
PRIMARY KEY (`ds_id`),
FOREIGN KEY (disease) REFERENCES disorder(disorder_name) WHERE (Disorder.disorder_type='Disease'),
FOREIGN KEY (syndrome) REFERENCES disorder(disorder_name) WHERE (Disorder.disorder_type='Syndrome')
)
My understanding is that checking values (e.g. ='Disease') requires a check constraint whereas linking to the original table requires a foreign key constraint. I cannot find any docummentation or YouTube tutorials detailing using BOTH these constraints simultaneously. As it has been very hard to find any examples of this code I wondered whether I have made a mistake with respect to database design but cannot think of a good alternative.
Can check and foreign key constraints be used together like this?
Thanks for your time!
QUESTION EDITED AS ORIGINALLY CONTAINED INFORMATION ABOUT TRYING TO ENFORCE THIS RELATIONSHIP AT THE DJANGO SIDE.

Answer 1

As I see the your MYSQL statement kinda worked, so all you need to do is add foreign key in the same way to the model like you did. Although this time, you will set 'null=True' and if you have unique values then 'unique=True' this will take care of empty columns. Now you have to implement rest in application logic while adding data, you have to be aware 'where disease type' , etc criteria. But when you are reading from the database Django will automatically prefetch related. Example query for reading data.
data=Diseasesyndromelink.objects.filter(disease__feild="query")
print(data.disease) #your foreign key object is prefetched here already.
Check out the documentation for more examples.
https://docs.djangoproject.com/en/3.0/topics/db/models/