DataSet select parent rows where child rows contains x value

Question

I have two DataTables in a DataSet linked together by a DataRelation and I'm trying to select all parent rows that have a child row with value x.

The parent table contains products and the child table contains categories that the product is in.

DataSet dsProducts = new DataSet();

DataTable dtProducts = new DataTable("products");
dtProducts.Columns.Add("entity_id", typeof(int));
dtProducts.Columns.Add("sku", typeof(string));
dtProducts.Columns.Add("mpn", typeof(string));
dtProducts.Columns.Add("brand", typeof(string));
dtProducts.Columns.Add("name", typeof(string));
dtProducts.Columns.Add("description", typeof(string));
dtProducts.Columns.Add("short_description", typeof(string));
dtProducts.Columns.Add("image", typeof(string));
dtProducts.Columns.Add("weight", typeof(double));
dtProducts.Columns.Add("qty", typeof(double));
dtProducts.Columns.Add("cost", typeof(double));
dtProducts.Columns.Add("price", typeof(double));
dtProducts.PrimaryKey = new DataColumn[] { dtProducts.Columns["entity_id"] };
dsProducts.Tables.Add(dtProducts);

DataTable dtCategories = new DataTable("categories");
dtCategories.Columns.Add("entity_id", typeof(int));
dtCategories.Columns.Add("category_id", typeof(int));
dsProducts.Tables.Add(dtCategories);

dsProducts.Relations.Add(new DataRelation("entity_id", dtProducts.Columns["entity_id"], dtCategories.Columns["entity_id"]));

---

EDIT

I've pieced together this piece of Linq code which works but seems pointless having a DataRelation

var rows = from prods in dsProducts.Tables["products"].AsEnumerable()
            join cats in dsProducts.Tables["categories"].AsEnumerable() on prods.Field<int>("entity_id") equals cats.Field<int>("entity_id")
            where cats.Field<int>("category_id") == id
            select prods;

Answer 1

Try:

DataRow[] rows = dsProducts.Tables["products"].Select("entity_id=" + id);

instead of:

DataRow[] rows = dsProducts.Tables["products"].Select("Child(entity_id).category_id = " + id);

Because you already create relation between two table with common column "entity_id",so as long as you entity_id is matched, you get your desired result.