Projectables with Setters are hydrated when pulling full entities #224

In Projectables 6.x, a settable [Projectable] property is populated when an untracked query materializes the whole entity. In Projectables 3.x, the property remained null unless the query used it in an expression.

For example:

    var posts = await db.Posts
        .AsNoTracking()
        .ToListAsync();

With a settable projectable:

    [Projectable(UseMemberBody = nameof(DisplayTitleExpression))]
    [NotMapped]
    public string? DisplayTitle { get; set; }

    private string? DisplayTitleExpression =>
        Blog == null ? null : Blog.Name + ": " + Title;

The query computes DisplayTitle and assigns it during materialization, even when the caller only needs the entity’s other properties.

This can add significant SQL and materialization work to whole-entity queries, especially when projectables contain correlated subqueries or other expensive expressions. Consumers may never read the resulting values.

Making the property read-only is not a suitable workaround for every application. HotChocolate's [UseProjection] needs setters to populate projectable properties. #129 Describes the HotChocolate requirements.

Expected behavior

Keep settable projectables available for query expressions and HotChocolate projections, while allowing consumers to opt out of populating them when a query materializes a whole entity.

A configuration option such as UseProjectables(o => o.PopulateSettableProperties(false)) could provide that control.

#84 #86 Introduced the automatic hydration behavior