How to combine two flat lists into one nested object

Question

I have three lists which contains three same properties in each collection. I want to combine a result into one collection. Ex classes structure is as below

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Answer 1

What are you describing sounds like two standard multi-key LINQ group joins. They are quite efficient (LINQ to Objects implementation uses prepared fast hash based lookups), so no further optimizations are needed:

var orderDetails = (
    from o in data.Orders
    join p in data.PaymentDetails
        on new { o.ProductId, o.CustomerId, o.OrderId }
        equals new { p.ProductId, p.CustomerId, p.OrderId }
        into orderPaymentDetails
    join c in data.CouponUsageDetails
        on new { o.ProductId, o.CustomerId, o.OrderId }
        equals new { c.ProductId, c.CustomerId, c.OrderId }
        into orderCouponUsageDetails
    select new OrderDetails
    {
        ProductId = o.ProductId,
        CustomerId = o.CustomerId,
        OrderId = o.OrderId,
        // Few other Properties of OrderDetail
        PaymentDetail = orderPaymentDetails.ToList(),
        CouponUsageDetail = orderCouponUsageDetails.ToList(),
    })
    .ToList();