EF数据库上下文未清除

Question

I have a program using Entity Framework where I'm dynamically selecting from a series of tables to display data. My tables within the database are named something along the lines of:

Revisions2016
Revisions2017
Revisions2018

So far, I'm able to select different tables using Fluent API through calling:

modelBuilder.Entity<Revision>().ToTable("Revisions" + suffix);

within the OnModelCreating function of my database context. My problem is that whenever I try creating a new database context through the following:

public static void AccessRevisions(string tableName)
{
  using (var context = new RevisionsContext(tableName))
  {
    var revisionListing = context.Revisions.ToList();
    foreach (object o in revisionListing)
    {
      Revision r = o as Revision;
      Console.WriteLine(String.Format("ID: {0} NOTES: {1} AUTHOR: {2}", r.RevisionID, r.RevisionNotes, r.RevisionAuthor));
    }
  }
}

Although I've specified the lifetime of the database context as limited through "using", it always seems to end up selecting the old table rather then changing to the specified table on calling the function. So say if I specified 2016 and then 2017, the query results always seem to be 2016.

I've tried forcing the initialisation of the context through using context.Database.Initialize(force: true); and Database.SetInitializer(new DropCreateDatabaseAlways<RevisionsContext>()); , but these don't seem to do anything. Would anyone be able to point me in the right direction on how to properly re-create my database context or dynamically switch tables?

Answer 1

So after a while of searching, I think I've found a potential solution to my problems. I'll post what I've found here to help others who've been searching for the solution to the same problem I have. With help from the blog post here by Benny Michielsen, I've constructed my context class as the following:

  public class VAMPModelContext : DbContext
  {
    private VAMPModelContext(string _ConnectionString)
      : base(_ConnectionString)
    {
      Database.SetInitializer<VAMPModelContext>(null);
    }

    private VAMPModelContext(DbCompiledModel compiledModel, string connectionString)
      : base(compiledModel)
    {
      Database.SetInitializer<VAMPModelContext>(null);
      Database.Connection.ConnectionString = connectionString;
    }

    public static VAMPModelContext CreateContext(string suffix)
    {
      string connectionString = ConfigurationManager.ConnectionStrings["VAMPTest"].ConnectionString;
      VAMPModelContext dummyContext = new VAMPModelContext(connectionString);

      DbModelBuilder builder = new DbModelBuilder(DbModelBuilderVersion.Latest);
      builder.Configurations.Add(new EntityTypeConfiguration<VAMPModel>());

      // Construct model mapping for ORM
      string tableName = "VAMPModels" + suffix;
      builder.Entity<VAMPModel>().ToTable(tableName);

      // Compile ORM object, hard link connection
      DbConnection dummyConnection = dummyContext.Database.Connection;
      DbCompiledModel compiledModel = builder.Build(dummyConnection).Compile();

      // Finally make our database context
      dummyContext = new VAMPModelContext(compiledModel, connectionString);

      return dummyContext;
    }

    public virtual DbSet<VAMPModel> ModelsList { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
      // Adjust field properties for model
      modelBuilder.Entity<VAMPModel>()
          .Property(e => e.ModelCode)
          .IsUnicode(false);

      modelBuilder.Entity<VAMPModel>()
          .Property(e => e.PriceAUD)
          .HasPrecision(18, 0);

      base.OnModelCreating(modelBuilder);
    }
  }

Although we can only call OnModelCreating once through normal constructors, there's another constructor that accepts a compiled model to generate the context from scratch. Hence, if we construct our own model and pass it through, we can in a sense "re-construct" the database context. From this class, we can simply call on it like this:

using(var context = VAMPModelContext.CreateContext("2016"))
{
  var modelsListing = context.ModelsList.ToList();
  foreach (object o in modelsListing)
  {
    VAMPModel m = o as VAMPModel;
    Console.WriteLine(String.Format("MODEL ID: {0} MODEL CODE: {1} PRICE: {2}", m.ModelID, m.ModelCode, m.PriceAUD));
  }
  context.Dispose();
}

I haven't tested the effects of all CRUD operations, but insertion and querying seems to work just fine. Hope this helps anyone who's been searching for a solution to the Entity-Framework dynamic table mapping approach I've been.