Fix ModelProducer to handle multiple APIs with different DbContexts

Fixes OData#527.

src/Microsoft.Restier.Providers.EntityFramework/Model/ModelProducer.cs

@@ -62,7 +62,19 @@ public Task<IEdmModel> GetModelAsync(ModelContext context, CancellationToken can
             var dbContext = context.GetApiService<DbContext>();
 
             var efModel = (dbContext as IObjectContextAdapter).ObjectContext.MetadataWorkspace;
-            var efEntityContainer = efModel.GetItems<EntityContainer>(DataSpace.CSpace).Single();
+            var efEntityContainers = efModel.GetItems<EntityContainer>(DataSpace.CSpace);
+            var efEntityContainer = efEntityContainers.FirstOrDefault(c => c.Name == dbContext.GetType().Name);
+            if (efEntityContainer == null)
+            {
+                if (efEntityContainers.Count > 1)
+                {
+                    var containerNames = efEntityContainers.Aggregate("", (current, next) => next.Name + ", ");
+                    throw new Exception("This project has multiple EntityFrameworkApis using different DbContexts, and the correct contect could not be loaded. \r\n" +
+                        $"The contexts available are '{containerNames.Substring(0, containerNames.Length - 2)}' but the Container expects '{efEntityContainer.Name}'.");
+                }
+                throw new Exception("Could not find the correct DbContext instance for this EntityFrameworkApi. \r\n" + 
+                    $"The Context name was '{dbContext.GetType().Name}' but the Container expects '{efEntityContainer.Name}'.");
+            }
             var itemCollection = (ObjectItemCollection)efModel.GetItemCollection(DataSpace.OSpace);
 
             foreach (var efEntitySet in efEntityContainer.EntitySets)