I am using dynamic linq on a project but don’t know how to run a particular query based on parameters. The example below shows the dynamic linq used to run 3 separate queries where a user has entered a filter expression or filer and sort expression etc. Without using if statements to check I (which seems incorrect and messy as I could end up with lots of if statements to cover the many different permutations ) I am unsure of how to generate the statements dynamically. Is this possible? I am running this query against Entity Framework.
context.Users.AsQueryable().Where(filterExpression)
context.Users.AsQueryable().Where(filterExpression).OrderBy(sortExpression)
context.Users.AsQueryable()Take(10).Where(filterExpression).OrderBy(sortExpression)
That should be achieved using expression tree only or may be using if..else clause
How to: Use Expression Trees to Build Dynamic Queries (C# and Visual Basic)
Related
I have a requirement which is somewhat similar to this. My API supports a filter where there is more than 8 filter parameter. So, I want to create a query dynamically based on the filter parameter passed. I am using CriteriaBuilder to create a dynamic query.
Now, I am able to create dynamic queries successfully but the issue comes when the user wants to sort on an aggregate functions. In my query, I have 4 aggregate (count) function. So to support sorting on these columns, I just use the expression of this aggregate function but what I want is to use the alias of this expression
Repeating the expression in select and order by doesn't seem right to me. So, is there a solution/workaround to the problem. I want to declare the expression alias once and use it in both select and order by clause and if required in my group by clause in future
You probably need to build a custom projection class, you can find a decent and easy one, SQLProjectionWithAliasSupport in https://hibernate.atlassian.net/browse/HHH-2952 (from Sergey Pulyaev)
If then one wants to support also sub criteria, things get more complicate
Looking at Dynamic Linq, it's possible to use strings to define the key parts of the query. My question is, is it possible to pass the entire query in as a string?
ie: var foo = "from..."
LINQ stands for "Language INtegrated Queries" - it's compiled with the rest of the code, not parsed on the run. You can use Microsoft.CSharp.CSharpCodeProvider to compile your query on the run - but you will have to know in advance what local objects you want to send to the query.
That's not possible in dynamic LINQ -- dynamic LINQ only replaces specific pieces of the query. You can use ExecuteQuery on the data context in LINQ to SQL or SqlQuery on a DbSet<T>, though, to execute specific SQL commands.
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 :)
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)