I thought that the purpose of using Linq2Nibernate was to return IQueryable and build up an expression tree. I am not sure if this is correct or not.
Why would anyone use Linq2Nibernate if they are not going to return IQueryable?
What else would it be used for?
I would love some more input on this topic
Linq For Nhibernate
I'm planning to use NHibernate.Linq to replace my HQL and criteria API queries with LINQ expressions. In other words, I'll generate the query in code (as a LINQ expression) and then pass it to NHibernate.Linq and NHib to convert it into a database query.
FYI there is an alpha version available.
I have planed to start using Linq2Nibernate but haven't got round to i yet.
My reason for wanting to user Linq2Nibernate is the nice syntax when constructing criterions and later querying them out.
Here is a nice simple example.
http://ayende.com/Blog/archive/2007/03/16/Linq-for-NHibernate.aspx
I am using Linq2Nhibernate with my repository pattern that returns IQueryable objects.
As you know, IQueryable is only a query definition - doesn't touch database yet.
Then local requirements are added to the query and finally the object or list is materialized.
Seems to work excellent and prevents unnecessary db queries for partial data at higher abstract layers.
What's Linq2NHibernate? As there are several projects which tried to implement a linq provider for nhibernate but all stopped before reaching completion.
Any linq provider needs to return IQueryable, or better an IEnumerable as that's how linq works. It's convenient to return an IQueryable as you then can re-use existing code to pad additional operators to an already created query (as ctx.Employee which might return IQueryable is already a query)
Related
I have a datatable (say myNameDT) which has data like this
Expected result:
When sector is Price, if it has fund value and component OTHER THAN "Active Return Contribution, It should update the Fundvalue in respective sector of Component Active Return Contribution. same applied for Index Value. Note: After update it should delete the unnecessary rows.
The result should look like,
Need as LINQ only. I have tried something as naive LINQ developer which is not fruitful. Thank you
What you're looking for is not feasible in a single LINQ statement. This is imposed not by LINQ itself but by the concept of SQL that LINQ relies on to perform operations on data. One of the things LINQ does, it lets you chain operations on the same dataset by combining them in a single query, however SELECTs cannot be combined with UPDATEs or DELETEs.
Finally, if what you're looking for is only syntactic sugar - this is not possible. If you are looking for a way to organize a set of data operations in a single TRANSACTION, you may want to look at these questions that give a hint on how LINQ interprets transaction operations:
TransactionScope vs Transaction in LINQ to SQL
How to create a LINQ to SQL Transaction?
Is there any way to use LIKE operator in LINQ to XML? I did try Googling but it results for Like operator in Linq to SQL not XML.
If yes, please recommend some reference. Thanks a lot in advance.
LINQ to XML is simply LINQ to Objects. LINQ to Objects means that you run .NET code over collections and you can use .NET code to do the filtering. In .NET we can use String.Contains, String.StartsWith and String.EndsWith to check whether a string is part of another string.
I have an IQueryable custs, a Customer cust, a CustomerComparer custCp which implements IEqualityComparer.
When I call custs.Contains(cust, custCp) I get an exception:
System.NotSupportedException: Unsupported overload used for query operator 'Contains'
But when I call custs.AsEnumerable().Contains(cust,custCp) it works. Can anyone explain why?
An operation on an IQueryable (in the case of Linq to Entities) is translated to SQL and executed in the database. Since you have an IEqualityComparer written in C#, the comparer can't be translated to SQL and the overload can't be supported.
When you translate it to an IEnumerable using AsEnumerable(), all the data is transferred from the database to memory, where the overload can be easily supported. The downside of course being that you're transferring more data than necessary (potentially the whole table) to have it filtered in memory.
IQueryable is implemented by query providers and under the hood translated into something the target system can understand. For example, IQueryable.Contains might be translated to a x IN y SQL expression.
On the other hand, IEnumerable is not translated like that. Its Contains works on anything that implements IEnumerable properly.
Using the IQueryable operators are executed server-side and therefore will gain you some performance as the query is executed remotely, whereas IEnumerable is executed locally (client-side). This means that if you were to get a database table, convert it to enumerable and then apply Contains on it, the whole table needs to be downloaded to your computer to be enumerated.
I am new to Linq please guide me on some basic things.
In read some articles on Linq. Some authers fill data in var from Linq query, some fills list of custom type objects and some fills data in IEnumerable and some do it in IQuryable. I could not get what is difference in these 4 and which one should be used in which situation.
I want to use Linq to SQL. What should I use?
Well, you can never declare that a method returns var - it's only valid for local variables. It basically means "compiler, please infer the static type of this variable based on the expression on the right hand side of the assignment operator".
Usually a LINQ to Objects query will return an IEnumerable<T> if it's returning a sequence of some kind, or just a single instance for things like First().
A LINQ to SQL or EF query will use IQueryable<T> if they want further query options to be able to build on the existing query, with the added bits being analyzed as part of the SQL building process. Alternatively, using IEnumerable<T> means any further processing is carried out client-side.
Rather than focusing on what return type to use, I suggest you read up on the core concepts of LINQ (and the language enhancements themselves, like var) - that way you'll get a better feel for why these options exist, and what their different use cases are.
I am creating a method that can create filter understood by NHibernate (by filter i mean a set of ICriteria object for example) from my abstract filter object.
public static IEnumerable<ICriterion> ToNhCriteria(this MyCriteria criteria)
{
// T4 generated function
// lots of result.Add(Expression.Or(Expression.Eq(),Expression.Eq)) expression trees - hard to generate
// Is there a way to generate HQL/Linq query here istead?
}
then i want to do something like
session.CreateCriteria<Entity>().Add(myCriteria.ToNhCriteria())
to filter entities.
The problem is that using Expression. methods (Expression.Or etc) is quite tedious (the method is generated and i have multiple or statements that have to be joined into an expression somehow).
Is there a way to avoid using Expression.Or() and create ICrietrion / ICriteria using LINQ or HQL?
Hey, did you check out this question? It shows going from Linq to NHibernate to a MultiCriteria (and on the way transforms a linq query to an ICriteria)
No that is not possible. Why don't you use linq instead of criteria?
Linq is not the best solution unless you want to do filtering on collection-side not on datbase-side using WHERE clauses.
Ayende suggests that ICriteria API is well suited for dynamic filter creation, the problem i had with multiple ORs has been tackled by using Restrictions.Disjunction()... that simplified a lot
At the time I asked the question I just didn't realize such things exist in NHibernate :)