score:2
Accepted answer
If the Reviews property contains all Reviews for a particular Book this is simple:
var result = books.Select(book => new
{
book.Title,
Rating = book.Reviews.Average(review => review.Rating)
});
But if you have separate collections you will need to perform a join:
var result = books.GroupJoin(reviews, // One-to-many
book => book.Id, // Primary key
review => review.Book.Id, // Foreign key
(book, reviews) => new // Selection
{
book.Title,
Rating = reviews.Average(review => review.Rating)
});
Source: stackoverflow.com
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