Bogus, AutoFixture, others(?): How to fill a nested model with fake data and set rules for specific properties?

Question

I have a very nested model that I want to create thousands of with fake data. But, also, some properties in the model need to be in a specific range or have specific rules. I looked at these two fake data generators:

AutoFixture only seems to generate fake data for everything.

Bogus can set rules like ranges for properties but all other properties remain null - or you have to define rules for all of them.

Did I miss something or is it not possible to fill the models with fake data and only set rules for specific properties?

Answer 1

AutoFixture enables you to establish rules for properties, either in a property-by-property basis, or by convention.

Customise a specific property

You can use Customize to change the behaviour for a particular type, including properties:

[Fact]
public void CustomizeSpecificProperty()
{
    var fixture = new Fixture();
    fixture.Customize<MyClass>(c => c.With(mo => mo.Number, 42));

    var actual = fixture.Create<MyClass>();

    Assert.Equal(42, actual.Number);
}

This particular customization changes the rule for all MyClass.Number properties; the value will always be exactly 42.

Customize by convention

You can also match various properties by convention, often by looking at a combination of property type and name:

[Fact]
public void CustomizeTextPropertyByConvention()
{
    var fixture = new Fixture();
    fixture.Customizations.Add(new TextPropertyBuilder());

    var actual = fixture.Create<MyClass>();

    Assert.Equal("Foo", actual.Text);
}

This option also requires that you write a custom TextPropertyBuilder class:

public class TextPropertyBuilder : ISpecimenBuilder
{
    public object Create(object request, ISpecimenContext context)
    {
        var pi = request as PropertyInfo;
        if (pi == null || pi.Name != "Text" || pi.PropertyType != typeof(string))
            return new NoSpecimen();

        return "Foo";
    }
}

This rule will apply to all string properties called "Text", no matter on which class they're defined.

AutoFixture comes with a rich API that will enable you express many of such rules in a more succinct manner, but these are the main building blocks.

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Both the above examples use this MyClass:

public class MyClass
{
    public int Number { get; set; }

    public string Text { get; set; }
}