I have been trying to load single items from with LLBLGen by fields in the table that are not primary keys.
I can only work out how to filer on primary keys on FetchEntity.
To filter on non primary keys I am having to getthe collection and use linq to get the first.
It fells like a smell, I was wondering if there was a better way?
public BinLocationEntity GetDefaultBinLocation(string firstName, string lastName)
{
var persons = new EntityCollection<PersonEntity>();
var filter = new RelationPredicateBucket();
filter.PredicateExpression.Add(PersonFields.FirstName == firstName);
filter.PredicateExpression.Add(PersonFields.LastName== lastName);
using (var adapter = this.DataAccessAdapter)
{
adapter.FetchEntityCollection(persons , filter);
}
return persons .First();
}
I know the demo code would be bad in real world, Its just there as an example.
You can also fetch by unique constraint:
https://www.llblgen.com/Documentation/5.6/LLBLGen%20Pro%20RTF/Using%20the%20generated%20code/Adapter/gencode_usingentityclasses_instantiating.htm#using-a-unique-constraints-value
It does not make a lot of sense to (directly) fetch a single entity using fields that are not primary keys and are not unique constraints. The generated code has no way to know that your query should logically result in a single entity being returned.
Using Linq .First() in these cases is completely appropriate and not at all a code/design smell.
The only thing that i'd add to your answer is that if you truly do expect that your query is going to return just a single result, that you change your FetchEntityCollection() call to:
adapter.FetchEntityCollection(persons , filter, 1);
to specifically limit the results to 0 or 1 rows. It might well be that the first result is the one you want, but without any top limit supplied it could be that this query is returning thousands of rows or more which can incur a huge performance hit.
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.
I have the following structure that I wan't to query using Linq, specifically Linq to Entities (Enitity Framework).
Table1: RouteMeta
Table2: SitePage
Multiple SitePages can link to the same RouteMeta.
I'm querying the Route Meta to select a number of rows. I'm using a generic repository, currently like this:
return r.Find().ToList();
There's nothing special about it - the Find method accepts an optional linq expression, so I could do something like this:
return r.Find(x => x.Status=1).ToList();
However, what I actually want to do is to select rows from RouteMeta where at least one linked row exists in SitePages with a property IsPublished = true.
return r.Find(x => x.SitePages("where y => y.IsPublished = true");
Obviously, the above isn't correct, I'm just trying to explain the scenario better.
Any advice appreciated.
try something like
return r.Find(x=>x.Sitepages.Any(y=>y.Published))?
I'd also suggesting using a profiler if possible to check that this translates properly into SQL. It probably should do but it depends on how your repository works.
Entity - AllSalesTerritory contains List<MySalesPerson> representing one to many relationship. I have Sql query to fetch the data where the two entities are mapped using a column TerritoryId. I use a following code to fill the entity using Dapper micro ORM:
List<AllSalesTerritory> allSalesTerrotories = _connection.Query<AllSalesTerritory, MySalesPerson, AllSalesTerritory>
(query, (pd, pp) =>
{
pd.SalesPersons.Add(pp);
return pd;
}, splitOn: "BusinessEntityId")
.ToList();
BusinessEntityId is the beginning column for SalesPerson entity on executing the Sql statement
Challenge that I face is, this kind of code helps in easily filling one to one relation, here I get just one value in each List<MySalesPerson>, instead of aggregating those values in the collection, essentially the same result as that of SQL join query. I can easily resolve the issue using a simple foreach loop and aggregating the values for MySalesPerson. However, I want to figure out:
Can Dapper automatically help me achieve it, tried few extensions, but they did not work as expected
Can a Linq code do it for me, since this is somewhat reverse of a SelectMany on an entity with one to many relationship
You can use a dictionary to keep track of the unique AllSalesTerritory objects. Assuming that the TerritoryId property is an int this would work.
var territories = new Dictionary<int, AllSalesTerritory>()
_connection.Query<AllSalesTerritory, MySalesPerson, AllSalesTerritory>
(query, (pd, pp) =>
{
AllSalesTerritory territory;
if(!territories.TryGetValue(pd.TerritoryId, out territory))
{
territories.Add(pd.TerritoryId, territory = pd);
}
territory.SalesPersons.Add(pp);
return territory;
}, splitOn: "BusinessEntityId");
List<AllSalesTerritory> allSalesTerrotories = territories.Values.ToList();
Basically what happens here is that Dapper will return one AllSalesTerritory and one MySalesPerson for each row in the results of your query. We then use a dictionary to see if the current AllSalesTerritory (pd) has been seen before based on the TerritoryId. If so then the local territory variable is assigned the reference to that object. If not then we assign pd to territory and then add that to the dictionary. Then we just add the current MySalesPerson (pp) to the territory.SalesPersons list.
I have a serious(it's getting me crazy) problem with LINQ to SQL. I am developing an ASP.NET MVC3 application using c# and Razor in Visual Studio 2010.
I have two database tables, Product and Categories:
Product(Prod_Id[primary key], other attributes)
Categories((Dept_Id, Prod_Id) [primary keys], other attributes)
Obviously Prod_Id in Categories is a foreign key. Both classes are mapped using the Entity Framework (EF). I do not mention the context of the application for simplicity.
In Categories there are multiple rows containing the Prod_Id. I want to make a projection of all Distinct Prod_Id in Categories. I did it using plain (T)SQL in SQL Server MGMT Studio according to this (really simple) query:
SELECT DISTINCT Prod_Id
FROM Categories
and the result is correct. Now I need to make this query in my application so I used:
var query = _StoreDB.Categories.Select(m => m.Prod_Id).Distinct();
I go to check the result of my query by using:
query.Select(m => m.Prod_Id);
or
foreach(var item in query)
{
item.Prod_Id;
//other instructions
}
and it does not work. First of all the Intellisense when I attempt to write query.Select(m => m. or item.shows just suggestions about methods (such as Equals, etc...) and not properties. I thought that maybe there was something wrong with Intellisense (I guess most of you many times hoped that Intellisense was wrong :-D) but when I launch the application I receive an error at runtime.
Before giving your answer keep in mind that;
I checked many forums, I tried the normal LINQ to SQL (without using lambdas) but it does not work. The fact that it works in (T)SQL means that there is something wrong with the LINQ to SQL instruction (other queries in my application work perfectly).
For application related reasons, I used a List<T> variable instead of _StoreDB.Categories and I thought that was the problem. If you can offer me a solution without using a List<T> is appreciated as well.
This line:
var query = _StoreDB.Categories.Select(m => m.Prod_Id).Distinct();
Your LINQ query most likely returns IEnumerable... of ints (judging by Select(m => m.Prod_Id)). You have list of integers, not list of entity objects. Try to print them and see what you got.
Calling _StoreDB.Categories.Select(m => m.Prod_Id) means that query will contain Prod_Id values only, not the entire entity. It would be roughly equivalent to this SQL, which selects only one column (instead of the entire row):
SELECT Prod_Id FROM Categories;
So when you iterate through query using foreach (var item in query), the type of item is probably int (or whatever your Prod_Id column is), not your entity. That's why Intellisense doesn't show the entity properties that you expect when you type "item."...
If you want all of the columns in Categories to be included in query, you don't even need to use .Select(m => m). You can just do this:
var query = _StoreDB.Categories.Distinct();
Note that if you don't explicitly pass an IEqualityComparer<T> to Distinct(), EqualityComparer<T>.Default will be used (which may or may not behave the way you want it to, depending on the type of T, whether or not it implements System.IEquatable<T>, etc.).
For more info on getting Distinct to work in situations similar to yours, take a look at this question or this question and the related discussions.
As has been explained by the other answers, the error that the OP ran into was because the result of his code was a collection of ints, not a collection of Categories.
What hasn't been answered was his question about how to use the collection of ints in a join or something in order to get at some useful data. I will attempt to do that here.
Now, I'm not really sure why the OP wanted to get a distinct list of Prod_Ids from Categories, rather than just getting the Prod_Ids from Projects. Perhaps he wanted to find out what Products are related to one or more Categories, thus any uncategorized Products would be excluded from the results. I'll assume this is the case and that the desired result is a collection of distinct Products that have associated Categories. I'll first answer the question about what to do with the Prod_Ids first, and then offer some alternatives.
We can take the collection of Prod_Ids exactly as they were created in the question as a query:
var query = _StoreDB.Categories.Select(m => m.Prod_Id).Distinct();
Then we would use join, like so:
var products = query.Join(_StoreDB.Products, id => id, p => p.Prod_Id,
(id,p) => p);
This takes the query, joins it with the Products table, specifies the keys to use, and finally says to return the Product entity from each matching set. Because we know that the Prod_Ids in query are unique (because of Distinct()) and the Prod_Ids in Products are unique (by definition because it is the primary key), we know that the results will be unique without having to call Distinct().
Now, the above will get the desired results, but it's definitely not the cleanest or simplest way to do it. If the Category entities are defined with a relational property that returns the related record from Products (which would likely be called Product), the simplest way to do what we're trying to do would be the following:
var products = _StoreDB.Categories.Select(c => c.Product).Distinct();
This gets the Product from each Category and returns a distinct collection of them.
If the Category entity doesn't have the Product relational property, then we can go back to using the Join function to get our Products.
var products = _StoreDB.Categories.Join(_StoreDB.Products, c => c.Prod_Id,
p => p.Prod_Id, (c,p) => p).Distinct();
Finally, if we aren't just wanting a simple collection of Products, then some more though would have to go into this and perhaps the simplest thing would be to handle that when iterating through the Products. Another example would be for getting a count for the number of Categories each Product belongs to. If that's the case, I would reverse the logic and start with Products, like so:
var productsWithCount = _StoreDB.Products.Select(p => new { Product = p,
NumberOfCategories = _StoreDB.Categories.Count(c => c.Prod_Id == p.Prod_Id)});
This would result in a collection of anonymous typed objects that reference the Product and the NumberOfCategories related to that Product. If we still needed to exclude any uncatorized Products, we could append .Where(r => r.NumberOfCategories > 0) before the semicolon. Of course, if the Product entity is defined with a relational property for the related Categories, you wouldn't need this because you could just take any Product and do the following:
int NumberOfCategories = product.Categories.Count();
Anyway, sorry for rambling on. I hope this proves helpful to anyone else that runs into a similar issue. ;)
I want to return a single row from the users table using domain account id as my primary and unique key
However when i use singleordefault and see its sql translation it performs entire select * from Users
my query is..
var user = base.SingleorDefault(t=>t.domainaccountid)
i want this to return just one row!
What is base ? Is it possible that you've coerced it to IEnumerable<T> at some point, rather than IQueryable<T>? that would cause this. Note that database composition is only possible when using IQueryable<T>, so if any of your methods have returned something other than this, composition will end.
You could try Where along with FirstOrDefault:
var user = base.Where(t => t.domainaccountid == 123).FirstOrDefault();
Try
var user = base.SingleorDefault(t=>t.domainaccountid==123);
SingleOrDefault looks for unique entries, it should be doing:
SELECT TOP 2 * FROM TABLE
It does this so that if it finds 2 results it will throw an exception as it is not unique.
If you don't care about finding it as a unique object, as you have other measure in place to prevent duplicates; or just don't care you can use FirstOrDefault in the following way:
array.FirstOrDefault(x => x.id == someOtherId);
This will perform the following:
SELECT TOP 1 * FROM TABLE
This will get your results quicker, especially in larger tables, because it will return as soon as the first result is found.