I'm doing the following:
public MyItem FetchSingleItem(int id)
{
string query = "SELECT Something FROM Somewhere WHERE MyField = {0}";
IEnumerable<MyItem> collection = this.ExecuteQuery<MyItem>(query, id);
List<MyItem> list = collection.ToList<MyItem>();
return list.Last<MyItem>();
}
It's not very elegant really and I was hoping there's something a little better to get a single item out using DataContext. I'm extending from DataContext in my repository. There's a valid reason why before you ask, but that's not the point in this question ;)
So, any better ways of doing this?
Cheers
If it is SQL Server, change your SQL to:
SELECT TOP 1 Something FROM Somewhere ...
Or alternatavely, change these lines
List<MyItem> list = collection.ToList<MyItem>();
return list.Last<MyItem>();
into this one:
return collection.First();
myDataContext.MyItem.Where(item => item.MyField == id)
.Select(item => item.Something)
.FirstOrDefault();
The record returned is undefined, since you have no ORDER BY. So it's hard to do an exact translation. In general, though, reverse the order and take the First():
var q = from s in this.Somewhere
where s.MyField == id
orderby s.Something desc
select s.Something;
return q.First();
Relational tables are unordered. So if you don't specify the record you want precisely, you must consider the returned record as randomly selected.
Related
I refer to this example: Return selected specified columns
Quote:
If BlobDetails isn't the LINQ entity, then you can do it directly:
var qry = from b in dc.Blobs
orderby b.RowVersion descending
select new BlobDetails {
Id = b.Id, Size = b.Size,
Signature = b.Signature, RowVersion = b.RowVersion};
return qry.ToList();
I see that they are selecting specific column in a query through the ORM-tool LINQ TO SQL.
Critics of ORM-tools say that, if I remember correctly, that ORM-tools select and return entire objects from the table, and limits the options of selecting only specific columns as one can do through classic SQL-programming. Of course, I have my doubts about that when I see this example, but nevertheless, I still keep asking myself the question: Does the database return only the selected columns, or does it return the entire objects, leaving the column-filtering to the ORM-tool?
From this example, they also have a class called Blobdetails:
public class BlobDetails
{
public int Id { get; set; }
public string Signature { get; set; }
public int Size { get; set; }
public System.Data.Linq.Binary RowVersion { get; set; }
}
Do I need to create my own classes everytime I only wish to select a few columns from a table through LINQ?
You don't need to create new classes to select few columns from a table. You can use anonymous types for that.
var qry = from b in dc.Blobs
orderby b.RowVersion descending
select new { b.Id, b.Size, b.Signature, b.RowVersion};
return qry.ToList();
Only selected columns are transferred. There is no difference between using plain SQL and using LINQ to SQL. When you are executing LINQ query, it is converted to plain SQL and executed. Then result is mapped to your objects.
You can use SQL Server Profiler to see what query was generated and executed on server. Also you can use LINQPad to see what SQL will be generated from your query. In your case query will be same either you use BlobDetails or anonymous object:
SELECT [t0].[Id], [t0].[Size], [t0].[Signature], [t0].[RowVersion]
FROM [Blobs] AS [t0]
ORDER BY [t0].[RowVersion] DESC
when you do projections LINQ does indeed only select those columns and there is nothing preventing you from materializing it however you want. So in your example code
select new BlobDetails
{
Id = b.Id,
Size = b.Size,
Signature = b.Signature,
RowVersion = b.RowVersion
};
Only b.id, b.size, b.signature, & b.rowversion are selected. You can verify this with sql profiler or your debugger, I seem to recall there is also a function you can call on the datacontext to get the last query that was ran.
I think that the answer to your first question is already in the POST you mentioned. However...
If your BlobDetails is not LINQ entity you can simply use it in your select statement to define (shrink) your projection attributes. For example:
var qry = from b in dc.Blobs
select new BlobDetails { Id = b.Id, Size = b.Size }
would compile to SQL query like SELECT Id, Size FROM Blob ....
But if BlobDetails is LINQ entity you will need to use that AsEnumerable() hack otherwise you will get NotSupportedException: Explicit construction of entity type in query is not allowed.
var qry = from b in dc.Blobs.AsEnumerable()
select new BlobDetails { Id = b.Id, Size = b.Size }
Edit
As #Chris Pitman stated in his comment this AsEnumerable() approach could create serious bottleneck, beacause the whole table would be loaded in memory before applying the projection. So it is not recommended!
To your second question:
You will need to create custom class for objects that you want use easily outside the scope of the method. Properties of an anonymous object are visible only in the scope, where they have been declared and anonymous objects can be cast only to type object.
So if you want to return anonymous objects from method the return type would has to be an enumerable of object or dynamic as #xeondev stated in his comment.
There's no need to create your own classes, you can return an anonymous type. You can write something like this
var qry = from b in dc.Blobs
orderby b.RowVersion descending
select new {
Id = b.Id, Size = b.Size,
Signature = b.Signature, RowVersion = b.RowVersion};
return qry.ToList();
Although the signature of the method should look to something like this
public IEnumerable<object> GetItems()
or
public dynamic GetItems()
So if you are going to use the result of linq query in outer scope like you example suggest, it is highly recommended you create your own classes.
I need some help in building the correct query. I have an Employees table. I need to get a list of all employees, that EENO (Employee ID) contains a string from a supplied array of partial Employee IDs.
When I use this code
// IEnumerable<string> employeeIds is a collection of partial Employee IDs
IQueryable<Employee> query = Employees;
foreach (string id in employeeIds)
{
query = query.Where(e => e.EENO.Contains(id));
}
return query;
I will get something like:
SELECT *
FROM Employees
WHERE EENO LIKE '%1111111%'
AND EENO LIKE '%2222222%'
AND EENO LIKE '%3333333%'
AND EENO LIKE '%4444444%'
Which doesn't make sense.
I need "OR" instead of "AND" in resulting SQL.
Thank you!
UPDATE
This code I wrote using PredicateBuilder works perfectly when I need to include these employees.
var predicate = PredicateBuilder.False<Employee>();
foreach (string id in employeeIds)
{
var temp = id;
predicate = predicate.Or(e => e.EENO.Contains(temp));
}
var query = Employees.Where(predicate);
Now, I need to write an opposite code, to exclude these employees,
here it is but it is not working: the generated SQL is totally weird.
var predicate = PredicateBuilder.False<Employee>();
foreach (string id in employeeIds)
{
var temp = id;
predicate = predicate.And(e => !e.EENO.Contains(temp)); // changed to "And" and "!"
}
var query = Employees.Where(predicate);
return query;
It's supposed to generate SQL Where clause like this one:
WHERE EENO NOT LIKE '%11111%'
AND NOT LIKE '%22222%'
AND NOT LIKE '%33333%'
But it's not happening
The SQL generated is this: http://i.imgur.com/9MDP7.png
Any help is appreciated. Thanks.
Instead of the foreach, just build the IQueryable once:
query = query.Where(e => employeeIds.Contains(e.EENO));
I'd take a look at http://www.albahari.com/nutshell/predicatebuilder.aspx. This has a great way of building Or queries, and is written by the guy that wrote LinqPad. The above link also has examples of usage.
I believe you can use Any():
var query = Employees.Where(emp => employeeIds.Any(id => id.Contains(emp.EENO)));
If you don't want to use a predicate builder, then the only other option is to UNION each of the collections together on an intermediate query:
// IEnumerable<string> employeeIds is a collection of partial Employee IDs
IQueryable<Employee> query = Enumerable.Empty<Employee>().AsQueryable();
foreach (string id in employeeIds)
{
string tempID = id;
query = query.Union(Employees.Where(e => e.EENO.Contains(tempID));
}
return query;
Also keep in mind that closure rules are going to break your predicate and only end up filtering on your last criteria. That's why I have the tempID variable inside the foreach loop.
EDIT: So here's the compendium of all the issues you've run across:
Generate ORs instead of ANDS
Done, using PredicateBuilder.
Only last predicate is being applied
Addressed by assigning a temp variable in your inner loop (due to closure rules)
Exclusion predicates not working
You need to start with the correct base case. When you use ORs, you need to make sure you start with the false case first, that way you only include records where AT LEAST ONE predicate matches (otherwise doesn't return anything). The reason for this is that the base case should just get ignored for purposes of evaluation. In other words false || predicate1 || predicate2 || ... really is just predicate1 || predicate2 || ... because you're looking for at least one true in your list of predicates (and you just need a base to build on). The opposite applies to the AND case. You start with true so that it gets "ignored" for purposes of evaluation, but you still need a base case. In other words: true && predicate1 && ... is the same as predicate1 && .... Hope that addresses your last issue.
I am stuck with a problem here. I am trying to compare items in a list to another list with much more items using linq.
For example:
list 1: 10,15,20
list 2: 10,13,14,15,20,30,45,54,67,87
I should get TRUE if all the items in list 1 occur in list 2. So the example above should return TRUE
Like you can see I can't use sequenceEquals
Any ideas?
EDIT:
list2 is actually not a list it is a column in sql thas has following values:
<id>673</id><id>698</id><id>735</id><id>1118</id><id>1120</id><id>25353</id>.
in linq I did the following queries thanks to Jon Skeets help:
var query = from e in db
where e.taxonomy_parent_id == 722
select e.taxonomy_item_id;
query is IQueryable of longs at this moment
var query2 = from e in db
where query.Contains(e.taxonomy_item_id)
where !lsTaxIDstring.Except(e.taxonomy_ids.Replace("<id>", "")
.Replace("</id>", "")
.Split(',').ToList())
.Any()
select e.taxonomy_item_id;
But now I am getting the error Local sequence cannot be used in LINQ to SQL implementation of query operators except the Contains() operator.
How about:
if (!list1.Except(list2).Any())
That's about the simplest approach I can think of. You could explicitly create sets etc if you want:
HashSet<int> set2 = new HashSet<int>(list2);
if (!list1.Any(x => set2.Contains(x)))
but I'd expect that to pretty much be the implementation of Except anyway.
This should be what you want:
!list1.Except(list2).Any()
var result = list1.All(i => list2.Any(i2 => i2 == i));
Customer.text is a field in an T-SQL DB (that I do not control and thus may not alter) of type "text".
I'd like to do something like this:
List<string> compare = new List<string>();
compare.Add("one");
compare.Add("two");
var q = from t in customer
where t.text.Contains( compare.First())
select t;
this will work.
But now I'd like to do something like: (!NOT WORKING!)
var q = from t in customer
where compare.Contains( t.text )
select t;
How can I achieve this? Is it even possible?
EDIT: The problem is obviously not exactly clear: A text column in SQL cannot be queried using "=" but only with LIKE. Thus the compare.Contains( t.text ) will result in an error, as it is converted into a query using "=".
What I did not tell - I thought it is irrelevant - is, that I use LINQ-to-ORM (LLBLGen in this case).
What I tried instead:
var q = from t in customer
where compare.Any( x => t.text.Contains(x) )
select t;
Now this did not work also. Currently I'm not at work, but the exception was something with a ConstantExpression not being convertable into a SetExpression.
I hope this gave some clarification.
EDIT2:
Joseph pointed this out to me: PredicateBuilder. It creates an Expression on a given ObjectType.
Now my problem is, that my type is an anonymous type out of multiple joins.
Is there an easy or elegant way to handle this?
Now I might be missing something, but your code looks like it should work. Did you include the namespaces at the top of the file?
using System.Linq;
using System.Linq.Expressions;
You could also rewrite it without the Linq2Sql syntax, like:
var q = customer.Where(c => compare.Contains(c.text));
You could build your query using LinqKit's free predicate builder class. Here is a blog post which describes its use and has a link to the download site.
http://thecodeslinger.wordpress.com/2008/10/28/linqkit-predicatebuildert-goodness/
Below is a code sample from the post
//First get a list of keywords that match the description entered.
string[] parts = txtInclude.Text.Split(new[] {‘ ‘});
string[] noparts = null;
if(txtButNot.Text.Trim().Length > 0)
noparts = txtExclude.Text.Trim().Split(new[] {‘ ‘});
var pred = PredicateBuilder.True<Pet>();
//here is where you would loop through your compare object
parts.ForEach(p => pred = pred.And(pl => pl.description.Contains(p)));
if(noparts != null)
noparts.ForEach(p => pred = pred.And(pl => !pl.description.Contains(p)));
var pets = from s in db.Pets.Where(pred)
select s;
You have to convert the text field to string
var query = from t in dataContext.table
where compare.Contains(t.textField.ToString())
select t;
I use codeigniter and have an issue about SELECT MAX ... I couldnot find any solution at google search...
it looks like it returns only id :/ it's giving error for other columns of table :/
Appreciate helps, thanks!
Model:
function get_default()
{
$this->db->select_max('id');
$query = $this->db->getwhere('gallery', array('cat' => "1"));
if($query->num_rows() > 0) {
return $query->row_array(); //return the row as an associative array
}
}
Controller:
$default_img = $this->blabla_model->get_default();
$data['default_id'] = $default_img['id']; // it returns this
$data['default_name'] = $default_img['gname']; // it gives error for gname although it is at table
To achieve your goal, your desire SQL can look something like:
SELECT *
FROM gallery
WHERE cat = '1'
ORDER BY id
LIMIT 1
And to utilise CodeIgniter database class:
$this->db->select('*');
$this->db->where('cat', '1');
$this->db->order_by('id', 'DESC');
$this->db->limit(1);
$query = $this->db->get('gallery');
That is correct: select_max returns only the value, and no other column. From the specs:
$this->db->select_max('age');
$query = $this->db->get('members');
// Produces: SELECT MAX(age) as age FROM members
You may want to read the value first, and run another query.
For an id, you can also use $id = $this->db->insert_id();
See also: http://www.hostfree.com/user_guide/database/active_record.html#select
CodeIgniter will select * if nothing else is selected. By setting select_max() you are populating the select property and therefore saying you ONLY want that value.
To solve this, just combine select_max() and select():
$this->db->select('somefield, another_field');
$this->db->select_max('age');
or even:
$this->db->select('sometable.*', FALSE);
$this->db->select_max('age');
Should do the trick.
It should be noted that you may of course also utilize your own "custom" sql statements in CodeIgniter, you're not limited to the active record sql functions you've outlined thus far. Another active record function that CodeIgniter provides is $this->db->query(); Which allows you to submit your own SQL queries (including variables) like so:
function foo_bar()
{
$cat = 1;
$limit = 1;
$sql = "
SELECT *
FROM gallery
WHERE cat = $cat
ORDER BY id
LIMIT $limit
";
$data['query'] = $this->db->query($sql);
return $data['query'];
}
Recently I have been utilizing this quite a bit as I've been doing some queries that are difficult (if not annoying or impossible) to pull off with CI's explicit active record functions.
I realize you may know this already, just thought it would help to include for posterity.
2 helpful links are:
http://codeigniter.com/user_guide/database/results.html
http://codeigniter.com/user_guide/database/examples.html