EF 学习笔记

No-Tracking Queries

When a database context retrieves table rows and creates entity objects that represent them, by default it keeps track of whether the entities in memory are in sync with what's in the database. The data in memory acts as a cache and is used when you update an entity. This caching is often unnecessary in a web application because context instances are typically short-lived (a new one is created and disposed for each request) and the context that reads an entity is typically disposed before that entity is used again.

You can disable tracking of entity objects in memory by using the AsNoTracking method. Typical scenarios in which you might want to do that include the following:

• A query retrieves such a large volume of data that turning off tracking might noticeably enhance performance.
• You want to attach an entity in order to update it, but you earlier retrieved the same entity for a different purpose. Because the entity is already being tracked by the database context, you can't attach the entity that you want to change. One way to handle this situation is to use the AsNoTracking option with the earlier query.

For an example that demonstrates how to use the AsNoTracking method, see the earlier version of this tutorial. This version of the tutorial doesn't set the Modified flag on a model-binder-created entity in the Edit method, so it doesn't need AsNoTracking.