The 2.1 release of [Entity Framework Core][efcore] brought the ability to do [value conversions][vc]. This is implemented through an abstract class, `ValueConverter`, which you can implement to convert a data type. They also provided [several built-in converters][bic] that you don't have to write, such as storing `enum`s as strings. To use a value converter, you provide a new instance of it and attach it to a property in your model's `OnModelCreating` event.
F# provides an `Option<'T>` type as a way to represent a value that may or may not be present. There are many benefits to defining optional values as `'T option` rather than checking for null; you can [read all about it][opt] if you'd like.
As I was working on a project, I already used `Option.ofObj` to convert my possibly-null results from queries to options; at the field level, though, I was working with default values. Could I use this new feature to handle `null`able columns as well? As it turns out, yes!
Here is the code for the value converter.
{% codeblock lang:fsharp %}
module Conversion =
open Microsoft.FSharp.Linq.RuntimeHelpers
open System
open System.Linq.Expressions
let toOption<'T> =
<@ Func<'T, 'T option>(fun (x : 'T) -> match box x with null -> None | _ -> Some x) @>
The `Conversion` module contains the functions that we'll need to provide in the `ValueConverter` constructor. _(With the way class inheritance is coded in F#, and the way `ValueConverter` wants its expressions in its constructor, this is a necessary step. I would have liked to have seen a no-argument constructor and overridable properties as an option, but I'm not complaining; this is a really great feature.)_ Within those functions, we make use of [code quotations][quot], then convert the quotation expressions to Linq expressions.
One other note; in the `toOption` function, if we used `Option.ofObj` instead of `box x`, the code would not support value types. This means that things like an `int option` field wouldn't be supported.
Now that we have our option converter, let's hook it into our model. In my project, each entity type has a static `configureEF` function, and I call those from `OnModelCreating`. Here's an abridged version of one of my entity types:
This line of code finds the type within the model, the property within the type, and provides the new instance of our option converter to it. In this entity, a `None` here indicates that the member uses the group's default e-mail format; `Some` would indicate that they've specified which format they prefer.
That's all there is to it! Define the coverter once, and plug it in to all the optional fields; now we have nullable fields translated to options by EF Core. ["Magic unicorn,"][mu] indeed!
_(Credits: Many thanks to Jiří Činčura for the [excellent value conversion blog post][vcblog] and Tomas Petricek for his [Stack Overflow answer on converting quotation expressions to Linq expressions][so].)_
