Is it possible to query all objects that have one or more possible children using NHibernate?

Question

I have a table called Recipes which contain one recipe per row. I also have a table called RecipeIngredients which contain one ingredient as used by a particular recipe. Thus, each Recipe row has one or more children RecipeIngredients rows.

What I'm trying to do is create a query to find all recipes that contain any ingredients in a list of desired ingredients. For example, show me all recipes that use either flour, eggs, or bananas.

The SQL would look something like this:

SELECT * FROM Recipes r
   WHERE EXISTS (select 1 from RecipeIngredients where RecipeId = r.RecipeId and IngredientId = ANY (5, 10, 15) limit 1);

However, I'm having a tough time figuring out how to express this as a LINQ query, or using the .QueryOver<T> method. I don't want to hard code in the SQL since this needs to be database agnostic and I want the configured NHibernate dialect to generate the correct code.

Any ideas?

Answer 1

NHibernate has support for this SQL statements, called

• 15.8. Detached queries and subqueries,
• 16.8. Subqueries

The syntax would be like this:

var session = ...// get a ISession 

Reciepe reciepe = null; // this will be a reference to parent

// the SELECT inside of EXISTS
var subquery = QueryOver.Of<ReciepeIngredient>()
    // The PARENT handling here
    // the filter, to find only related ingredients
    .Where(item => item.ReciepeId == reciepe.ID)
    .Where(Restrictions.In("ID", new[] { 5, 10, 15 }))
    // Select clause
    .Select(ing => ing.ID)

    ;

Having the above subquery, we can use it like this

// the '() => reciepe' setting is essential here, it represents parent in a subquery
var query = session.QueryOver<Reciepe>(() => reciepe);

query.WithSubquery
    // our EXISTS (...
    .WhereExists(subquery);

var list = query
    .List<Reciepe>();

NOTE: let's check even more deeper subquery(ies) usage here Query on HasMany reference

Answer 2

A Few More Details:

Radim's answer turns out to be the best way to express the sub-query, however there's a few gotchas that took me a while to figure out. Thus, I'll post an answer as well to fill in the details.

First off, the line:

.Where(Restrictions.In("ID", new[] { 5, 10, 15 }))

Doesn't actually work if ID refers to an entity itself. In other words:

.Where(Restrictions.In("Ingredient", arrayOfIds))

Will throw a very confusing null reference exception since the Ingredient field maps to a Ingredients object. Using "IngredientId" doesn't work either. In that case, you have to use this:

.Where(Restrictions.In("Ingredient", arrayOfIds
   .Select(id => new Ingredients(id)).ToArray()))

To cast the ID array to an array of Ingredients objects. After that, things start working.

I also found an easy performance improvement that made the query run noticably faster, at least on PostgreSQL. If you change the sub-query from:

WHERE exists (SELECT RecipeIngredientId FROM recipeingredients WHERE RecipeId = r.RecipeId and IngredientId in (:p0, :p1))

To:

WHERE exists (SELECT RecipeIngredientId FROM recipeingredients WHERE RecipeId = r.RecipeId and IngredientId in (:p0, :p1) LIMIT 1)

It will only have to check a single row within the nested query. The query ran about twice as fast for me. This is easy to express:

var subquery = QueryOver.Of<RecipeIngredients>()
   .Where(item => item.Recipe.RecipeId == recipe.RecipeId)
   .Where(Restrictions.In("Ingredient", allowedIngs))
   .Select(i => i.RecipeIngredientId).Take(1);

Hope this helps!

Answer 3

Try this Linq query:

        recipes.Where(r => r.RecipeIngredients.Any(i => new long[]{5, 10, 15}.Contains(i.Id)));