I currently have the following code:
var FirstNameList = db.Clients.Include(x => x.FirstNames).Include(x => x.Addresses).SelectMany(a => a.FirstNames).Where(x => x.Name.ToLower().Trim() == "Max".ToLower().Trim()).ToList();
I have a navigation property of FirstNames and Addresses which I which to include in the result.
I use the SelectMany Statement because it, for me, is the only one which works. Kind of. It returns all the FirstNames where the Name equals Max.
What I would like it to do is return all the Clients who have the property Name equals Max from the table FirstNames.
the other way I thought about doing this was to take all the ID's returned from FirstNameList and then returning the Clients by querying the data against the FirstNameList but I would be then querying the database twice, which seems inefficient.
My question is is it possible, and how would I go about querying the database to return my Clients, if it was?
Kind regards
The following query should give you what you're looking for. You can look within each client's FirstNames and see if any of them are named "max". (In this case, since "max" is a constant you're typing in, I removed the ToLower().Trim() from it)
var clientsNamedMax = db.Clients.Include(x => x.FirstNames).Include(x => x.Addresses).Where(x => x.FirstNames.Any(y => y.Name.ToLower().Trim() == "max")).ToList();
Related
I have a table that contains objects from type person, each person has a unique column of type string called id, and I have a hashset of Id's that I want to fetch the person's object for.
I thought I could do it efficiently by doing this:
HashSet<string> ids = //Some HashSet of ids
var idsHashSet = DbContext.Persons.Select(p => p.Id).ToHashSet();
idsHashSet.IntersectWith(ids);
var result = DbContext.Persons.Where(p => idsHashSet.Contains(p.Id)).ToList());
I saw in the database itself that the query being sent is:
SELECT <something>
FROM <something>
WHERE p.name IN ('id1', 'id2', id3'...)
And it's not efficient because I already know that each element in the HashSet is present in the database, so I wanted to know, is there a more efficient way to do it?
You can do this to shorten your code:
var result = DbContext.Persons
.Where(x => ids.Contains(x.Id))
.ToList();
But it will be converted to the exact same SQL query, though, which is efficient enough.
"And it's not efficient because I already know that each element in the HashSet is present in the database" I'm not sure what you mean by this....the db has no idea if all the ids are present or not, regardless of whether you do.
How large is the set of ids? Safer to create a temp table with the IDs and return a JOIN select if it's very large...otherwise I Don't see an issue with your query as it is.
SQL server is able to translate EF .First() using its function TOP(1). But when using Entity Framework's .Last() function, it throws an exception. SQL server does not recognize such functions, for obvious reasons.
I used to work it around by sorting descending and taking the first corresponding line :
var v = db.Table.OrderByDescending(t => t.ID).FirstOrDefault(t => t.ClientNumber == ClientNumberDetected);
This does it with a single query, but sorting the whole table (million rows) before querying...
Do I have good reasons to think there will be speed issues if I abuse of this technique ?
I thought of something similar... but it requires two query :
int maxID_of_Client = db.Where(t => t.ClientNumber == ClientNumberDetected).Max(t => t.ID);
var v = db.First(t => t.ID == maxID_of_Client);
It's consisting of retrieving the max ID of the client, then use this ID to retrieve the last line of the client.
It doesn't seems faster to query two times...
There must be a way to optimize this and use a single query without sorting millions of datas.
Unless there is something I don't understand, I'm probably not the first to think about this problem and I want to solve it for good !
Thanks in advance.
The assumption driving this question is that result sets with no ordering clause come back from your DB in any predictable order at all.
In reality, result sets that come back from SQL have no implicit ordering and none should be assumed.
Therefore, the result of
db.Table.FirstOrDefault(t => t.ClientNumber == ClientNumberDetected)
is actually indeterminate.
Whether you're taking first or last, without ordering it's all meaningless anyway.
Now, what goes to SQL where you add an ordering clause to your LINQ? It will be something similar to...
SELECT TOP(1) something FROM somewhere WHERE foo=bar ORDER BY somevalue
or, in the the descending/last case
SELECT TOP(1) something FROM somewhere WHERE foo=bar ORDER BY somevalue DESC
From SQL's POV, there's no significant difference here and your DB will be optimized for this sort of query. The index can be scanned in either direction, and the cost of each query above is the same.
TL;DR :
db.Table.OrderByDescending(t => t.ID)
.FirstOrDefault(t => t.ClientNumber == ClientNumberDetected)
is just fine.
I have a database of strings that contain IDs. I need to pass that list into a LINQ query so I can pull the correct records.
model.SelectedDealers = db.dealers.Any(a => a.sdealer_name.Contains(UserToEdit.UserViewAccesses.Select(s => s.ViewReferenceNumber)));
SelectedDealers is of type dealers
ViewReferenceNumber should be a list of strings which should match sdealer_name
So essentially I am trying to find all of the dealers whos sdealer_name matches the list of sdealer_names I have in my UserToEdit.UserViewAccesses
I've tried moving parts around and switching them in different spots and can't seem to figure this out.
Any() is just a boolean indicating if there are any results. It doesn't actually return the results.
If I understand what you are after correctly, then this might work:
var dealerNames = UserToEdit.UserViewAccesses.Select(s => s.ViewReferenceNumber).ToList();
model.SelectedDealers = db.dealers.Where(a => dealerNames.Contains(a.sdealer_name));
So essentially I am trying to find all of the dealers whos
sdealer_name matches the list of sdealer_names I have in my
UserToEdit.UserViewAccesses
var dealersthatmatched = (from d in UserToEdit.UserViewAccesses.sdealer_names
where d == sdealer_name
select d).ToList()
Wish I could have made a comment instead, but as I don't have enough rep I can't. I wish I understood the requirement better, but you seem ready and able to try stuff so perhaps you find this useful.
I'm using LINQ to search multiple fields on a single phrase and I'm using Contains() to do this. Up until today, when I noticed that the find method isn't working correctly.
So I'd like to be able to search each of the fields where there is any match. I've done some searching on Google and through SO and found suggestions of using Any()? Does anyone have any suggestions?
var l = _getData().Select(_marshallData).ToList();
return l.Where(x => x.Name.Contains(what) || x.Pumpkin.Contains(what) || x.Orange.Contains(what) || x.Tomato.Contains(what)).ToList();
Please excuse the stupid field names, I've had to change them for confidentiality.
Since you're materializing the data already you could do this:
var l = _getData().Select(_marshallData).AsEnumerable();
return l.Where(x => new[] { x.Name, x.Pumpkin, x.Orange, x.Tomato }.Any(s => s.Contains(what)));
But if you're using an ORM (like Entity Framework) this trick probably won't work (since there's a good chance this can't be converted to a SQL expression). In this case, you're original solution is fine, but it would probably be better to do this before you materialize the data.
return _getData()
.Where(x =>
x.Name.Contains(what) || x.Pumpkin.Contains(what) ||
x.Orange.Contains(what) || x.Tomato.Contains(what))
.Select(_marshallData)
.AsEnumerable();
Of course the field names here might be different, since the parameter x is probably a different type. Without further information, it's up to you to write this filter correctly.
Note that in both examples, I've eliminated the calls to ToList. Unless you absolutely need the result to a List<T> this is probably unnecessary.
I'd like to do a LINQ query that can compare multiple variables to a single string. I've seen LINQ for LIKE queries of array elements, and it's helpful, but not quite. I need the reverse.
What I'd like to do is the following: let's say I have a Company object with both Name and Address. I also have a string keyword. Then, I'd like to find all Companys in a list that have the keyword in either their Name or Address. In SQL it would be...
SELECT * FROM Company
WHERE Name LIKE '%keyword%' OR Address LIKE '%keyword%'
I've been using Entity Framework, and I've tried the following: context.Companies.Where(x => new string[] { x.Name, x.Address }.Contains(keyword), as well as context.Companies.Where(x => new string[] { x.Name, x.Address }.Any(r => r.Contains(keyword)), but neither were successful. The first one gives me an IN clause, and the second one... I don't know what it does, but it doesn't give me what I want.
I'm sorry I don't have a very in-depth understanding of Expressions (yet); I wished I was able to write my own custom Expressions from scratch, but scratch it I can't just yet... Can anybody help me with this?
Any reason for not just using the || operator?
context.Companies.Where(x => x.Name.Contains(keyword) ||
x.Address.Contains(keyword))
I'd expect this to be translated into your original SQL.