与EntityFramework中的独立关联一起实现“公用”外键

Question

I am using EntityFramework 6 in my project, which uses a AuditLog table to keep track of changes in multiple other tables (to simply, I name them T1, T2,... T5 without the details). Every record in AuditLog table is for one of the other table (T1, ... T5). So it is a [0..1] to many relationship.

I do not want to use the standard FK constraint based association, because it would introduce 5 FK in the AuditLog table, and 4 out of the 5 FK columns are null for each record. I'd like to use a "common" column in the AuditLog table to serve as a "common" FK to other tables, the value in this FK column could be the primary key from one of the 5 tables. As a result, the following SQL is generated for AuditLog table:

CONSTRAINT [FK_AuditLogT1] FOREIGN KEY ([EntityId]) REFERENCES [dbo].[T1] ([Id]),
CONSTRAINT [FK_AuditLogT2] FOREIGN KEY ([EntityId]) REFERENCES [dbo].[T2] ([Id]),
CONSTRAINT [FK_AuditLogT3] FOREIGN KEY ([EntityId]) REFERENCES [dbo].[T3] ([Id]),
CONSTRAINT [FK_AuditLogT4] FOREIGN KEY ([EntityId]) REFERENCES [dbo].[T4] ([Id]),
CONSTRAINT [FK_AuditLogT5] FOREIGN KEY ([EntityId]) REFERENCES [dbo].[T5] ([Id])

At runtime, when the app is adding new objects of type T1(or T2...T5), the EF would throw the following exception due to the FK constraint:

Exception: The INSERT statement conflicted with the FOREIGN KEY constraint "FK_AuditT2". The conflict occurred in database "xxxx", table "dbo.T2", column 'Id'.

I have tried to use independent association to map the relationship between AuditLog and other tables, there is no exception, but, EF will not auto-populate the entityId column in AuditLog table with the primary key of the other tables.

Is there a way I can use the independent association but also maintain the primary key of the other tables in the entityId in AuditLog?

Answer 1

You cannot do this with Entity Framework associations - this is a database limitation - a field cannot be in multiple relationships as the constraints will conflict (eg T1 has key 10, but this isn't present in T2, therefore the constraints on your "Key" field will be broken.

You've not said what you want to achieve in your audit log, but there are a number of efforts around to add an automatic audit log for Entity Framework entities - these would be simpler because you wouldn't have to explicitly add them, but may not fill all of your requirements. Some of these include:

• EntityFramework.Extended
• AuditDbContext

Plus a number of blog posts found by searching for Entity Framework Audit Log