Skip to main content

NHibernate eager fetching

NHibernate supports the concept of eager fetching for a long time. However there are some things to consider when you start using this with the out-of-the-box NHibernate Linq provider.
Let’s first look at the obvious way:
var customers = session
.Query<Customer>()
.Fetch(c => c.Orders)
.ToList();

This will return the customer and all the customer’s orders in a single SQL statement.
Rule #1: Fetch() statements must always come last.
If you want to mix Fetch with other clauses, Fetch must always come last. The following statement will throw an exception:
var customers = session
.Query<Customer>()
.Fetch(c => c.Orders)
.Where(c => c.CustomerId == "ABC")
.ToList();

But this will work fine:

var customers = session
.Query<Customer>()
.Where(c => c.CustomerId == "ABC")
.Fetch(c => c.Orders)
.ToList();

Rule #2: Don’t fetch multiple collection properties at the same time.
Be careful not to eagerly fetch multiple collection properties at the same time. The following statement will execute a Cartesian product query against the database, so the total number of rows returned will be the total Subordinates times the total orders.

var employees = session
.Query<Employee>()
.Fetch(e => e.Subordinates)
.Fetch(e => e.Orders)
.ToList();

Rule #3: Fetch grandchild collections using FetchMany.
You can fetch grandchild collections too.  The following statement will throw an exception:

var customers = session
.Query<Customer>()
.Fetch(c => c.Orders)
.Fetch(c => c.Orders.OrderLines)
.ToList();

But if you use ‘FetchMany’ and ‘ThenFetchMany’ it will work fine:

var customers = session
.Query<Customer>()
.FetchMany(c => c.Orders)
.ThenFetchMany(o => o.OrderLines)
.ToList();

Popular posts from this blog

DevToys–A swiss army knife for developers

As a developer there are a lot of small tasks you need to do as part of your coding, debugging and testing activities.  DevToys is an offline windows app that tries to help you with these tasks. Instead of using different websites you get a fully offline experience offering help for a large list of tasks. Many tools are available. Here is the current list: Converters JSON <> YAML Timestamp Number Base Cron Parser Encoders / Decoders HTML URL Base64 Text & Image GZip JWT Decoder Formatters JSON SQL XML Generators Hash (MD5, SHA1, SHA256, SHA512) UUID 1 and 4 Lorem Ipsum Checksum Text Escape / Unescape Inspector & Case Converter Regex Tester Text Comparer XML Validator Markdown Preview Graphic Color B

Help! I accidently enabled HSTS–on localhost

I ran into an issue after accidently enabling HSTS for a website on localhost. This was not an issue for the original website that was running in IIS and had a certificate configured. But when I tried to run an Angular app a little bit later on http://localhost:4200 the browser redirected me immediately to https://localhost . Whoops! That was not what I wanted in this case. To fix it, you need to go the network settings of your browser, there are available at: chrome://net-internals/#hsts edge://net-internals/#hsts brave://net-internals/#hsts Enter ‘localhost’ in the domain textbox under the Delete domain security policies section and hit Delete . That should do the trick…

Azure DevOps/ GitHub emoji

I’m really bad at remembering emoji’s. So here is cheat sheet with all emoji’s that can be used in tools that support the github emoji markdown markup: All credits go to rcaviers who created this list.