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So, I'm using EF DbContexts in the following way. One AppDbContext is an abstract class derived from DbContext, it contains my DbSets and is used as a type to inject into services. Its "implementation" is like AppDbContextMySql, which is self-explanatory - it's derived from AppDbContext and handles the connection to the actual DB. There can be several such abstract/implementation pairs to separate the data tables, but usually, all of them point to the same actual DB instance.

Then I need to migrate it all, so I add a MigrationDbContext implementing all the datasets and all the entity configurations needed, namely composite primary keys which can only be configured in OnModelCreating override.

The question is, if I already have the data model configuration in MigrationDbContext, have applied the migration successfully to the DB, and it's DB's job to handle keys and indexes anyway, do I need to have the model configuration in my actually consumed AppDbContext or AppDbContextMySql? In other words, is the model only used to generate migration scripts, or is it also needed at runtime to help EF handle the data in any way?

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The short answer is yes, the model is definitely needed in all the cases.

Generating migrations from it is just one of the possible usages, and for sure not the primary - migrations are optional feature which may not be used at all.

The primary purpose of the model is to provide the mappings between the data (objects, properties, navigations) and the storage (database tables, columns and relationships). It is basically the M part of the ORM (Object Relational Mapper) what EF Core is.

It controls all EF Core runtime behaviors - LINQ queries, change tracking, CUD etc. Like what is the associated table/column name, what is the PK/FK property/column, what is the cascade delete behavior, what is the cardinality of the relationship and many others.

The way you asked the question makes me think you are assuming that the model is used just for migrations while at runtime EF Core takes that metadata information from the actual database. That's not the case. For EF Core the database is whatever you told them in the model configuration. Whether the physical database is in sync (correctly mapped) is not checked at all. That's why people start getting errors for non exiting columns in their queries when they screw-up some fluent configuration, or more importantly - do not include such.

Which is the main drawback of using separate (a.k.a. "bounded") contexts for a single database. Because having a DbSet type property in the derived context is not the only way the entity is included in the model. In fact typed DbSet properties are just for convenience and are nothing more than shortcut to the DbContext.Set<T>() method. A common (a sort of hidden) method of including entity in the model is when it is referred (either directly or via collection) by another already included entity. And all that recursively.

And when the entity is included in the model, it needs the associated fluent configuration regardless of the concrete context class. Same for referenced entities, and their references etc.

So I don't really see the benefits of "bounded" context classes - they probably work for simple self containing object set with no relations to other (and vice versa), but can easily be broken by this automatic entity inclusion mechanism.

For reference, see Including types in the model and Creating and configuring a model.

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Now that you explained it, I realized it. EF at the very least should somehow be able to Find(...) an entity with its key, so the key should still be defined somewhere. Well, that means that either I need to maintain two similar configurations or not to use the bounded DbContexts approach.

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