Efficient way to get a huge number of records sorted via Linq. Also some help regarding editing an existing DB entry - asp.net-mvc

Question

The fist part of my question is
suppose I have a poco class
public class shop{
public virtual string fruitName {get;set;}
public virtual double numberOfFruitsLeftToConsume {get;set;}
public virtual double numberOfFruitsLeftForStorage {get;set;}
public virtual List<Locations> shopLocations {get;set;}
}
I add new fruits in the db by creating a new object of shop and then add it via my context then save it.
Now to retrieve the data
will it be more efficient for me to first filter by fruit name get a List then in that collection should I run my query to sort by the number of fruits to consume , or should I just put it all into one query. Supposing that the site has more than 1000 hits/sec and a massive DB, which method will be efficient.
List<shop> sh = context.shopDB.Where(p => p.fruitName == "mango" &&
p.fruitName == "apple").ToList();
List<shop> sh = sh.Where(f => f.numberOfFruitsLeftToConsume >= 100 &&
f.numberOfFruitsLeftForStorage <= 100).ToList();
The example has no meaning , I just wanted to show the type of query I am using.
The second part of my question is, when I initialize the class shop I do not initialize the List within it. Later on when I try to add it it does not get saved, the shop is connected to the user.
ApplicationUser user = await usemanager.FindByEmailAsync("email");
if(user.shops.shopLocations == null){
user.shops.shopLocation = new List<Location>();
uset.shops.shopLocation.Add(someLocation);
await context.shopDB.SaveChangesAsync();
}
////already tried
//List<Location> loc = new List<Location>();
//loc.Add(someLocation);
//user.shops.shopLocation = loc;
//await context.shopDB.SaveChangesAsync();
I tried both the methods in a try catch block and no exception is thrown.
If you need any more details or if something is not clear to you please ask.
Thank you.
If I add Location and LocationId properties to shop, and then save, I can only view the LocationId , but the Location property still remains null.
To clear any question , If I save a location Individually it saves. So I don't think I'm providing wrong data.

Answer 1

Now to retrieve the data will it be more efficient for me to first filter by fruit name get a List then in that collection should I run my query to sort by the number of fruits to consume , or should I just put it all into one query. Supposing that the site has more than 1000 hits/sec and a massive DB, which method will be efficient.
You are the only one who can answer that question by measuring the query performance. Only as a general rule I can say that putting all into one query and let the database do the most of the job (eventually tuning it by creating appropriate indexes) is usually preferable.
What about the second part of the question (which basically is a different question), this block
if (user.shops.shopLocations == null)
{
user.shops.shopLocation = new List<Location>();
user.shops.shopLocation.Add(someLocation);
await context.shopDB.SaveChangesAsync();
}
looks suspicious. Your shopLocations member is declared as virtual, which means it's intended to use lazy loading, hence most probably will never be null. And even if it is null, you need to keep only the new part inside the if and do the rest outside, like this
if (user.shops.shopLocations == null)
user.shops.shopLocation = new List<Location>();
user.shops.shopLocation.Add(someLocation);
await context.shopDB.SaveChangesAsync();

Answer 2

1st Question
Because you are calling .ToList() at the end of your queries it will have to fetch all the rows from the db each time, so it will be much faster to do all your filtering in one LINQ .Where() call like this:
List<shop> sh = context.shopDB.Where(p => p.fruitName == "mango" && p.fruitName == "apple" && f.numberOfFruitsLeftToConsume >= 100 && f.numberOfFruitsLeftForStorage <= 100).ToList();
but if you don't call .ToList() at the end of first Linq query, spliting your query into two calls will be tottally fine and will yield the same performance as the previous approach like this:
var sh = context.shopDB.Where(p => p.fruitName == "mango" &&
p.fruitName == "apple");
List<shop> shList = sh.Where(f => f.numberOfFruitsLeftToConsume >= 100 &&
f.numberOfFruitsLeftForStorage <= 100).ToList();
2nd Question
when you initialize the Location for the shop, you must set the shopId property and then it should work, if not the problem might be with your database relationships.