Filter list of entity objects by string child object property using Linq Lambda

Question

I am trying to return an IQueryable lands filtered by a child object property Owner.Name. Is working well with the query style solution, but I want to use a lambda one.

On short these are my classes mapped by EntityFramework:

public class Land
{
     public int Id { get; set; }

     public virtual ICollection<Owner> Owners { get; set; }
}

public class Owner
{
        public int Id { get; set; }
        public string Name { get; set; }
        public int LandId { get; set; }

        public virtual Land Lands { get; set; }
}

The query which is working fine:

 var list = from land in db.Lands
                   join owner in db.Owners on land.Id equals Owner.LandId
                   where owner.Name.Contains("Smit")
                   select land;

I was trying using this:

var list = db.Lands.Where(lnd => lnd.Owners.Count() > 0 &&
             lnd.Owners.Where(own => own.Name.Contains("Smit")).Count() > 0);

It works only for small lists, but for some with thousands of records it gives timeout.

It would be better to rearrange the data to a data structure like Lookup, which would make your querying many time faster, your current query has the time complexity of O(N^2), which is killing performance for lots of data, and for 1000 records complexity becomes 1 million iterations — Mrinal Kamboj
– Mrinal Kamboj, Commented May 15, 2019 at 12:48
In actual Query version, you are using a Join, then why would you not Join in the Lambda query, it is bound to have an impact — Mrinal Kamboj
– Mrinal Kamboj, Commented May 16, 2019 at 11:31

Gibbon · Accepted Answer · 2019-05-15 12:04:40Z

2

Well, one issue which may be causing the speed problem is that your lambda version and your non-lambda versions do very different things. You're non lambda is doing a join with a where on one side of the join.

Why not just write the lambda equivalent of it?

var list = db.Lands.Join(db.Owners.Where(x=> x.Name.Contains("Smit")), a=> a.Id, b => b.LandId, (a,b) => a).toList();

I mean, that is the more direct equivalent of your non lambda

answered May 15, 2019 at 12:04

Gibbon

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1 Comment

Mrinal Kamboj Over a year ago

This is the correct replacement of the OP's query version

hassan.ef · Accepted Answer · 2019-05-15 12:00:01Z

0

I think you can use this one:

var list = db.Lands.Where(lnd => lnd.Owners.Any(x => x.Name.Contains("Smit")));

answered May 15, 2019 at 12:00

hassan.ef

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Comments

Jacek Glen · Accepted Answer · 2019-05-15 12:30:50Z

0

Try something more straightforward:

var lands = db.Owners.Where(o => o.Name.Contains("Smit")).Select(o => o.Lands);

You just need to make sure that Owner.Name is not null and LINQ will do the rest.

answered May 15, 2019 at 12:30

Jacek Glen

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2 Comments

bucyDev Over a year ago

You re forgetting join by LandId

Jacek Glen Over a year ago

That relation should be taken care of by Entity Framework itself. The relationship can be defined using convention, like in the example above, or explicit using Fluent API.

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