I've got 4 unrelated entities and I would like to query them for a certain keyword. Each entity had a LastModified field and I would like to return the top 50 result of the search over the 4 tables order by the LastModified field. Is this even possible?
In the past I've used a view to do this kind of stuff... but I don't understand how to achieve it with EF Code First.
You will need to:
Search each table
Transform the results into a common type
Merge the results
Sort/select from this merged list
Take the first 50 results.
The first two can be done with LINQ to Entities, the latter three with LINQ to Objects.
EDIT This approach would look something like:
var resA = from a in db.A
where ConditionA(a)
select MakeSharedFromA(a);
var resB = from b in db.B
where ConditionB(b)
select MakeSharedFromB(b);
var resC = from c in db.C
where ConditionC(c)
select MakeSharedFromC(c);
var resD = from d in db.D
where ConditionD(d)
select MakeSharedFromD(d);
var merged = resA.AsEnumerable().Take(50)
.Concat(resB.AsEnumerable().Take(50))
.Concat(resC.AsEnumerable().Take(50))
.Concat(resD.AsEnumerable().Take(50));
var res = merged.Sort(x => x.SortField).Take(50);
If each of the MakeSharedFromX methods can be replaced by a lambda (to give an expression tree) which is limited to operators and functions that LINQ to Entities supports, then drop the AsEnumerable and Take calls from the concatenation step and all can be carried out server side.
Related
I need to retrieve data from 2 SQL tables, using LINQ. I was hoping to combine them using a Join. I've looked this problem up on Stack Overflow, but all the questions and answers I've seen involve retrieving the data using ToList(), but I need to use lazy loading. The reason for this is there's too much data to fetch it all. Therefore, I've got to apply a filter to both queries before performing a ToList().
One of these queries is easily specified:
var solutions = ctx.Solutions.Where(s => s.SolutionNumber.Substring(0, 2) == yearsToConsider.PreviousYear || s.SolutionNumber.Substring(0, 2) == yearsToConsider.CurrentYear);
It retrieves all the data from the Solution table, where the SolutionNumber starts with either the current or previous year. It returns an IQueryable.
The thing that's tough for me to figure out is how to retrieve a filtered list from another table named Proficiency. At this point all I've got is this:
var profs = ctx.Proficiencies;
The Proficiency table has a column named SolutionID, which is a foreign key to the ID column in the Solution table. If I were doing this in SQL, I'd do a subquery where SolutionID is in a collection of IDs from the Solution table, where those Solution records match the same Where clause I'm using to retrieve the IQueryable for Solutions above. Only when I've specified both IQueryables do I want to then perform a ToList().
But I don't know how to specify the second LINQ query for Proficiency. How do I go about doing what I'm trying to do?
As far as I understand, you are trying to fetch Proficiencies based on some Solutions. This might be achieved in two different ways. I'll try to provide solutions in Linq as it is more readable. However, you can change them in Lambda Expressions later.
Solution 1
var solutions = ctx.Solutions
.Where(s => s.SolutionNumber.Substring(0, 2) == yearsToConsider.PreviousYear || s.SolutionNumber.Substring(0, 2) == yearsToConsider.CurrentYear)
.Select(q => q.SolutionId);
var profs = (from prof in ctx.Proficiencies where (from sol in solutions select sol).Contains(prof.SolutionID) select prof).ToList();
or
Solution 2
var profs = (from prof in ctx.Proficiencies
join sol in ctx.Solutions on prof.SolutionId equals sol.Id
where sol.SolutionNumber.Substring(0, 2) == yearsToConsider.PreviousYear || sol.SolutionNumber.Substring(0, 2) == yearsToConsider.CurrentYear
select prof).Distinct().ToList();
You can trace both queries in SQL Profiler to investigate the generated queries. But I'd go for the first solution as it will generate a subquery that is faster and does not use Distinct function that is not recommended unless you have to.
At first you might think this is duplicate of this question but hopefully you will see it is not.
I also want to select groups of rows that are consecutive but consider that this time the entries are telephone numbers, therefore, stored as string.
I have been trying somethink like:
var numbers = await (from a in context.Telephones
from b in context.Telephones
Convert.ToInt32(a.Number) < Convert.ToInt32(b.Number) &&
Convert.ToInt32(b.Number) < (Convert.ToInt32(a.Number) + numberQuantity)
group b by new { a.Number }
into myGroup
where myGroup.Count() + 1 == numberQuantity
select myGroup.Key.Number).ToListAsync();
But this fails with:
LINQ to Entities does not recognize the method 'Int32 ToInt32(System.String)' method, and this method cannot be translated into a store expression.
I understand that LINQ to Entities does not support Convert.ToInt32 but I am running out of ideas here to make it work.
So if my database has:
2063717608
2063717609
2063717610
2063717611
2063717613
2063717614
How can I select consecutive rows based on the string values? And when querying for 3 consecutive numbers get results like:
From 2063717608 to 2063717610
From 2063717609 to 2063717611
1- If you are aware of performance side effect of calling AsEnumerable() cast your query and do conversion in memory on the retrieved entities.
2- If you don't want solution #1, you have to look for a way to solve the conversion problem:
2-1- Either change the column type in the database to int
2-2- Or select one of the solution previously proposed by other developers such as:
Problem with converting int to string in Linq to entities
I am trying to implement some search functionality within our app and have a situation where a User can select multiple Topics from a list and we want to return all activities that match at least one of the selected Topics. Each Activity can have 0-to-many Topics.
I can write a straight SQL query that gives me the results I want like so:
SELECT *
FROM dbo.ACTIVITY_VERSION av
WHERE ACTIVITY_VERSION_ID IN (
SELECT ACTIVITY_VERSION_ID
FROM dbo.ACTIVITY_TOPIC at
WHERE at.TOPIC_ID IN (3,4)
)
What I can't figure out is how to write a LINQ query (we are using Linq to Sql) that returns the same results.
I've tried:
activities.Where(x => criteria.TopicIds.Intersect(x.TopicIds).Any());
this works if activities is a list of in memory objects (i.e. a Linq to Objects query), but I get an error if I try to use the same code in a query that hits the database. The error I receive is:
Local sequence cannot be used in LINQ to SQL implementations of query operators except the Contains operator.
I believe that this means that Linq to Sql doesn't know how to translate either Intersect or Any (or possibly both). If that is the case, I understand why it isn't working, but I still don't know how to make it do what I want it to and my Google-fu has not provided me with anything that works.
Haven't tested it. But this is how you ll go about it.
List<int> IDs = new List<int>();
IDs.Add(3);
IDs.Add(4);
var ACTIVITY_VERSION_IDs = ACTIVITY_TOPIC
.Where(AT => IDs.Contains(AT.TOPIC_ID))
.Select(AT=>AT.ACTIVITY_VERSION_ID)
var results = ACTIVITY_VERSION
.Where(AV => ACTIVITY_VERSION_IDs.Contains(AV.ACTIVITY_VERSION_ID))
Given that I have three tables (Customer, Orders, and OrderLines) in a Linq To Sql model where
Customer -- One to Many -> Orders -- One to Many -> OrderLines
When I use
var customer = Customers.First();
var manyWay = from o in customer.CustomerOrders
from l in o.OrderLines
select l;
I see one query getting the customer, that makes sense. Then I see a query for the customer's orders and then a single query for each order getting the order lines, rather than joining the two. Total of n + 1 queries (not counting getting customer)
But if I use
var tableWay = from o in Orders
from l in OrderLines
where o.Customer == customer
&& l.Order == o
select l;
Then instead of seeing a single query for each order getting the order lines, I see a single query joining the two tables. Total of 1 query (not counting getting customer)
I would prefer to use the first Linq query as it seems more readable to me, but why isn't L2S joining the tables as I would expect in the first query? Using LINQPad I see that the second query is being compiled into a SelectMany, though I see no alteration to the first query, not sure if that's a indicator to some problem in my query.
I think the key here is
customer.CustomerOrders
Thats an EntitySet, not an IQueryable, so your first query doesn't translate directly into a SQL query. Instead, it is interpreted as many queries, one for each Order.
That's my guess, anyway.
How about this:
Customers.First().CustomerOrders.SelectMany(item => item.OrderLines)
I am not 100% sure. But my guess is because you are traversing down the relationship that is how the query is built up, compared to the second solution where you are actually joining two sets by a value.
So after Francisco's answer and experimenting with LINQPad I have come up with a decent workaround.
var lines = from c in Customers
where c == customer
from o in c.CustomerOrders
from l in o.OrderLines
select l;
This forces the EntitySet into an Expression which the provider then turns into the appropriate query. The first two lines are the key, by querying the IQueryable and then putting the EntitySet in the SelectMany it becomes an expression. This works for the other operators as well, Where, Select, etc.
Try this query:
IQueryable<OrderLine> query =
from c in myDataContext.customers.Take(1)
from o in c.CustomerOrders
from l in o.OrderLines
select l;
You can go to the CustomerOrders property definition and see how the property acts when it used with an actual instance. When the property is used in a query expression, the behavior is up to the query provider - the property code is usually not run in that case.
See also this answer, which demonstrates a method that behaves differently in a query expression, than if it is actually called.
Using HQL, you can build a query like the following which will return an array of objects for each row selected by simply using a comma delimited list in the select clause:
select mother, offspr, mate.Name
from Eg.DomesticCat as mother
inner join mother.Mate as mate
left outer join mother.Kittens as offspr
How would you do this with an ICriteria? Specifically, I am trying to build a paged query which also returns the total row count in the same query.
Ayende shows how you can do this with HQL (http://ayende.com/Blog/archive/2007/04/27/Paged-data--Count-with-NHibernate-The-really-easy-way.aspx). I need to replicate this using an ICriteria.
I would presume that the answer lies in prefetching the associations that you want to. That way you can fetch the needed part of your object graph in a single shot. You would do this in criteria queries like so.
ICriteria query = session.CreateCriteria(typeof (Cat))
.SetFetchMode("Mate", FetchMode.Join)
.SetFetchMode("Kittens", FetchMode.Join);
IList<Cat> results = query.List<Cat>();
This will give you back a list of cats with both the Mate and the Kittens prepopulated. You can then navigate to these properties without incurring an N+1 penalty. If you need a more flattened result I'd do it using linq, perhaps like this.
var results = query.List<Cat>()
.Select((c) => new {mother = c, mate = c.Mate, offspr = c.Kittens});
This will give you back a flattened list of anonymous types with the given properties. This will work if all you need is prefetching the object graph. However if you need to prefetch things such as counts or sums then you will need to examine the Projections and Alias parts of Criteria queries.
One more thing. If you are trying to exactly duplicate your above query, you can do so like this.
ICriteria query = session.CreateCriteria(typeof (Cat), "mother")
.CreateAlias("Mate", "mate", JoinType.InnerJoin)
.CreateAlias("Kittens", "offspr", JoinType.LeftOuterJoin)
.SetResultTransformer(Transformers.AliasToEntityMap);
This will return you basically the same as your hql query, however instead of using an indexed list it will use a dictionary that maps the alias to the entity.
You can convert your criteria to a count criteria by this way
DetachedCriteria countCriteria = CriteriaTransformer.TransformToRowCount(criteria); // criteria = your criteria
rowCount = countCriteria.GetExecutableCriteria(session).UniqueResult<int>();
Ayende also added Multi Criteria support: http://ayende.com/Blog/archive/2007/05/20/NHibernate-Multi-Criteria.aspx. That's in the 2.0 release, apparently.
You can also do paging by manipulating the resultset on the Criteria:
ICriteria query = session.CreateCriteria(typeof (Cat), "mother")
query.SetFirstResult(x); // x = page start
query.SetMaxResults(y); // y = page size
List results = query.List();