It is easy to do a Contains, StartsWith, or EndsWith query in Entity Framework when you know the pattern you'd like to find. But how to do the equivalent of "LIKE [Column]"?
For instance, this is easy:
Name LIKE '%Th'
But how do you do this? where Prefix is a column.
Name LIKE [Prefix]
I tried to use the SqlMethods and got the following error.
LINQ to Entities does not recognize the method 'Boolean Like(System.String, System.String)' method, and this method cannot be translated into a store expression.
I resolved this problem by using LINQ to Entities to bring back the set of data that I wanted, then using LINQ to Objects to filter the data.
I do not think there is currently a way to have a LINQ to Entities query be resolved in a database server as ...
... LIKE [column-name]
First, SqlMethods is for use in LINQ-to-SQL, not LINQ-to-Entities.
The best you can do to use the filter in the database engine (so you don't have to pull all data in memory first) is:
context.Entities.SqlQuery("SELECT * FROM EntityTable WHERE Name LIKE Prefix")
Related
i would like to know if you have any idea how i can achieve this, considering a query stored as string in the configuration file.
I tried to use SqlQuery applied to the DBSet, but the problem is that SqlQuery requires me to select all properties of the required entities in my query. If i don't consider any column, it will complain because is not able to map the query to the entities.
I don't want to select all properties of the entities i want to query.
Thanks
If you are using EF then why not use Database.ExecuteSqlCommand()? It's in the System.Data.Entity namespace.
For example:
int result = db.Database.ExecuteSqlCommand("Non SELECT SQL etc...");
Well, I ended up implementing a mechanism using reflection that basically receives a group of fields to select, and constructs dynamic objects with those fields, so when applied the query with the joins between the entities, will only bring the fields I am looking for.
So, considering Entity1, Entity2, Entity3 with the following relationship
<b>Entity1</b>{
<br/> Entity1Name, <br/> List<*Entity2*> Entity2Items, <br/> etc..
<br/>}
and
<b>Entity2</b> { <br/> Entity2Name, <br/> List<*Entity3*> Entity3Items <br/>}
I can store e.g. the following query in the configuration file, and retrieve the information:
"Entity1.Entity1Name", <br/>
"Entity1.Entity2Items.Entity2Name", <br/>
"Entity1.Entity2Items.Entity3Items.Entity3Name"
Anyway, I was just trying to see if there would be any solution out-of-the-box that would require minimal code changes.
Thank you.
I know that it is possible using #SqlResultSetMapping but I want to select not whole entity from the database but some fields and then map i to my entity using one of the constructor which accept that fields. Is that possible to map result with #EntityResult for only a few #FieldResult? I was trying to do that and all the time I get error which said that there is not specify mapping for some fields which exist in that entity.
The disadvantage of #SqlResultSetMapping is that you have to select all the columns.
The alternate way of doing this manually iterate over the DB result and populate your objects.
Well, if you are using JPA 1.0 your only option (not considering the manual mapping, of course), is to use #SqlResultSetMapping and map the whole table columns. With JPA 2.1 you can add a javax.persistence.ConstructorResult (see docs here) to map only the needed columns.
getting this error on where clause.
trying to fetch records from sql server with linQ query in entity framework.
var Stud = contextSchool.Student.Where(x => x.STDNT_ID == lstStudent[i].StudentID).FirstOrDefault();
if i store value of list in a variable and then use in where clause, it works but not with list.
complete error:
linQ to entities does not recognize the method Models.Repository.Student get_Item(Int32)' method, and this method cannot be translated into a store expression.
i also get this error on some other projects, the problem is that u cant use methods like get_Item or f.e. AddDays() in ur linq command.
So u kinda have to play around it.
U have to create an object (list, or whatever) before ur linq command and fill it with the data u need (per methods) and use the variable in ur linq command.
I'm currently working with ASP.NET Core and I want to use Automapper to map a Linq Expression. The mapping statement is:
var targetConditions = _mapper.Map<Expression<Func<Entity, bool>>>(filter);
where filter is a formal parameter in the form:
(Expression<Func<EntityDTO, bool>> filter
In the mapping profile I have the following map created:
CreateMap<Expression<Func<EntityDTO, bool>>, Expression<Func<Entity, bool>>>();
I'm using a generic repository pattern with EF. I want to get a list of DTOs filtered by, of course, DTO's fields from my controller. I then need to convert from DTO filter to entities filter in the Business Layer before doing any query using Linq for EF.
Even though the expression does get coverted from EntityDTO to Entity, the parameters in the lambda expressions inside don't, raising all sorts of errors when I further use it with EF. Any idea how can this be done?
Try this:
IQueryable<Customer> query = repository.getCustomers(); // entities query
query.ProjectTo<CustomerDto>().Where(dtoFilterExpression)
I am thinking of designing a business rule engine which basically generates an EF query from a set of string values stored in a database.
For e.g. I will store the connection string, table name, the where condition predicate, and select predicate as string fields in a db and would like to construct the EF query dynamically. For e.g.
var db = new DbContext(“connectionstring”);
var wherePredicate = Expression.FromString(“p => p.StartDate > new DateTime(2014,5,1))
var selectPredicate = Expression.FromString(“p => p”)
var results = db.Set(“Projects”).Where(wherepredicate).Select(selectPredicate)
For constructing the predicates I can use DynamicExpression or Dynamic LINQ library.
However how do I access db.Set(“Projects”) where Projects is the entity name and apply the where and select predicates? (or something like db[“Projects”].Where().Select).
I tried the non-generic version of the DbContext.Set(Type entityttype) method, however couldn’t figure out how to apply Where and Select predicates to the returned object.
I am trying to avoid generating SQL queries and instead rely on dynamically generated EF code.
This doesn't make much sense. You can create method that will work on string instead of generic type using reflection, but you'd have to return DbSet not DBSet<T>. And on that one you cannot execute LINQ's methods (basically), because there's no type (during compilation). Of course you can do it all the way using reflection, but then, why??? You're loosing 90% of what O/R mapper does for you.