c# - How to force fire OnModelCreating every DataContext initialized up

Question

I want to fire OnModelCreating every new DataContext(new Entity()) ... But when i create a connection of a table , it works. When create a connection for another table, OnModelCreating doesnt work again, so because i got error,

the entity type <tableName> is not part of the model for the current context.

public class DataContext : DbContext
{
    private BaseEntity _entity;

    public DataContext(BaseEntity entity)
    {
        Database.Connection.ConnectionString = Parameters.ConnectionString;

        _entity = entity;
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        _entity.Map(modelBuilder); // this is dynamic fluent api here
    }
}

Answer 1

ta.speot.is is right that OnModelCreating fires only once because modelBuilder is cached. There are several cases when OnModelCreating is required to execute again. For example when multi-tenancy is implemented across sessions then one may require to fire OnModelCreating again.

When modelBuilder is created first time, then EF caches it to increase performance. it is cached using IDbModelCacheKeyProvider.CacheKey. OnModelCreating fires when it does not find Cache associated with CacheKey. So to get OnModelCreating fired again IDbModelCacheKeyProvider.CacheKey must be changed.

To change cache key, DbContext class must implement IDbModelCacheKeyProvider. When new cache key is returned In CacheKey property of IDbModelCacheKeyProvider then OnModelCreating event fires again.

For example see following code block:

public class TenantContext : DbContext, IDbModelCacheKeyProvider
{
    string IDbModelCacheKeyProvider.CacheKey {
        get { return tenentID.ToString(); }
    }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
    }
}

When tenantID is changed, it forces OnModelCreating to execute because cache might not be available for new tenantID.

Note: CacheKey should be changed only if it is very urgent. Frequent change in CacheKey will decrease performance.