Entity-framework – What are the benefits of ORM lazy loading

entity-frameworknhibernateorm

I'm researching data layer underpinnings for a new web-based reporting system and have spent a lot of time evaluating ORM's over the last few days. That said, I've never dealt with "lazy loading" before and am confused at why its the default setting for LINQ queries in the Entity Framework. It seems like it creates a lot of network traffic and unnecessarily tasks the database with additional queries that could otherwise be resolved with joins.

Can someone describe a scenario in which lazy loading would be beneficial?

Some meta:

The new system will be working against a database with hundreds of tables and many terabytes of data in a production environment with over 3,000 concurrent users on the system 24 hours a day. They will be retrieving large datasets continuously. Is it possible that an ORM just isn't the right solution for our needs, especially since the app will be web-based?

Best Answer

When we talk about lazy loading we are talking about Navigation Properties (how we follow foreign keys). What lazy loading will do for us is to populate the entity from a remote table as we attempt to access that entity. For example if we have a model like this

public class TestEntity
{
    public int Id{get;set;}
    public AnotherEntity RemoteEntity{get;set;}
}

And call the following

var something = WhateverContext.TestEntities.First().RemoteEntity;

We will get 2 database calls, one for WhateverContext.TestEntities.First() and one for loading the remote entity.

I'm a web guy, (and more specifically an MVC guy) and for web stuff I don't think there is ever a good reason for wanting to do this, One database call is always going to be quicker than two if we require the same set of data.

The situation where I think that lazy loading is actually worth considering is when you don't know when you do your first query if you will need the second entity at all. In my opinion this is much more relevant for windows applications where we have a user who is performing actions in real time (rather than stateless MVC where users are requesting whole pages at once). For example I think lazy loading shines when we have a list of data with a details link, then we don't load the details until the user decides they want to see them.

I don't feel this extends to paging, sorting and filtering, IMO there should be one specifically crafted database query per page of data you are displaying, which returns exactly the data set required to display that page.

In terms of your performance question, I feel that EF (or another ORM) can probably meet your needs here but you want to be careful with how you are retrieving large datasets due to the way EF tracks entities. Check out my EF performance tuning cheat sheet, and read up on DetectChanges and AsNoTracking if you do decide to use EF with large queries.

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