Today I was working with a team that was implementing a CQRS architecture. CQRS (Command Query Responsibility Segregation) is a design pattern that separates the responsibilities of reading and writing data into distinct models. The idea is to use one model to handle commands (which modify data) and another model to handle queries (which retrieve data). This separation allows for better scalability, performance optimization, and flexibility, as the read and write operations can be independently optimized or scaled based on the specific needs of the system.
After creating a read model for a specific table in the database, EF core started to complain and returned the following error message:
System.InvalidOperationException : Cannot use table 'Categories' for entity type 'CategoryReadModel since it is being used for entity type 'Category' and potentially other entity types, but there is no linking relationship. Add a foreign key to 'CategoryReadModel' on the primary key properties and pointing to the primary key on another entity type mapped to 'Categories'.
As mentioned in the error above, both the Category entity and the CategoryReadModel where mapped to the same table 'Categories'. Here is the corresponding code:
It turns out that EF Core doesn’t support mapping to entities to the same table what explains the error message above.
So mapping both a read and write model in this way seems to be a problem…
How to fix this?
One way to fix this, is to create 2 separate DbContext; one that is used for your read model and a separate one that is used for your write models. The advantage of this approach is that it provides clear separation at the cost of an increased application complexity.
In this case we decided to go for a simpler alternative. As the read model is a readonly version of the table, we updated the mapping and indicated that the Categories table should be handled like a view for the CategoryReadModel.
Here is the updated code: