Question

I’ve been having trouble enforcing my data model in MySQL and I was wondering if there are any better ways to do this.

Model:

Platforms <- Has many groups

Groups <- Has one platform, Has many SymbolSets

SymbolSets <- Belongs to many groups

My problem is that SymbolSets can only belong to groups that have the same platform. So if Platform A has Group A with SymbolSet A then Platform B with Group B shouldn’t be able to get SymbolSet A, but any group in Platform A should be able to get it.

Right now I have a link table between SymbolSets and Groups and I enforce the uniqueness per platform in code, but I’d rather have a solution enforced in the DB so nobody can go muck around with it and break things. I would prefer to not have this many to many relationship in the first place, but that doesn’t really fit our business model.

Answer 1

Platform     
----------
PlatformId
PRIMARY KEY (PlatformId)

Grouping        
----------
GroupingId
PlatformId
PRIMARY KEY (GroupingId, PlatformId)
FOREIGN KEY (PlatformId)            
  REFERENCES Platfrom(PlatformId)

SymbolSet         
----------
SymbolSetId 
PRIMARY KEY (SymbolSetId)

When a SymbolSet is to be assigned to a Group (a new row in SymbolSetToGroup), it first has to be assigned to the same Platform that the Group belong to (so a new row in SymbolSetAssigned first).

SymbolSetAssigned               --- assigned to Platform
-----------------
SymbolSetId 
PlatformId
PRIMARY KEY (SymbolSetId)
UNIQUE KEY (SymbolSetId, PlatformId)
FOREIGN KEY (SymbolSetId)                              
  REFERENCES SymbolSet(SymbolSetId)
FOREIGN KEY (PlatformId)            
  REFERENCES Platfrom(PlatformId)

SymbolSetToGroup
----------------
SymbolSetId 
GroupingId
PlatformId
PRIMARY KEY (SymbolSetId, GroupingId)
FOREIGN KEY (SymbolSetId, PlatformId)                              
  REFERENCES SymbolSetAssigned(SymbolSetId, PlatformId)
FOREIGN KEY (GroupingId, PlatformId)             
  REFERENCES Grouping(GroupingId, PlatformId)