I am very new to Mongo (this is actually day 1) and using the C# driver that is available for it. One thing that I want to know (as I am not sure how to word it in Google) is how does mongo handle executing queries when I want to grab a part of the collection.
What I mean by this is that I know that with NHibernate and EF Core, the query is first built and it will only fire when you cast it. So say like an IQueryable to IEnnumerable, .ToList(), etc.
Ex:
//Query is fired when I call .ToList, until that point it is just building it
context.GetLinqQuery<MyObject>().Where(x => x.a == 'blah').ToList();
However, with Mongo's examples it appears to me that if I want to grab a filtered result I will first need to get the collection, and then filter it down.
Ex:
var collection = _database.GetCollection<MyObject>("MyObject");
//Empty filter for ease of typing for example purposes
var filter = Builders<MyObject>.Filter.Empty;
var collection.Find(filter).ToList();
Am I missing something here, I do not think I saw any overload in the GetCollection method that will accept a filter. Does this mean that it will first load the whole collection into memory, then filter it? Or will it still be building the query and only execute it once I call either .Find or .ToList on it?
I ask this because at work we have had situations where improper positioning of .ToList() would result is seriously weak performance. Apologies if this is not the right place to ask.
References:
https://docs.mongodb.com/guides/server/read_queries/
The equivalent to your context.GetLinqQuery<MyObject>() would be to use AsQueryable:
collection.AsQueryable().Where(x => x.a == "blah").ToList();
The above query will be executed server side* and is equivalent to:
collection.Find(Builders<MyObject>.Filter.Eq(x => x.a, "blah")).ToEnumerable().ToList();
* The docs state that:
Only LINQ queries that can be translated to an equivalent MongoDB query are supported. If you write a LINQ query that can’t be translated you will get a runtime exception and the error message will indicate which part of the query wasn’t supported.
i have an editor where people can create custom sql-queries. those queries are NHibernate.IQuery objects.
in our project we are using NHibernate.Linq.LinqExtensionMethods.Query to get an IQueryable object which can be used to apply filters that are executed deferred (everything on the DB).
now i want to create another filter that is based on the custom sql-queries.
what i want is something like this:
queryable.Where(x=> <sql-query contains x>)
the only thing i can do right now is execute the sql-query beforehand and then filter the queryable with the resulting list of elements.
IList<T> elements = query.List<T>();
queryable.Where(x => elements.Contains(x)).ToList();
the problem with that approach is, that the list of elements can be huge. if its possible, i would like to perform the whole statement directly on the database, so that i dont have to transfer all objects to my application, then send all objects back to the database as the filter-parameters...
edit: the first query (the one yielding the list of elements to check for in the second query) is constructed from a plain sql-string as follows:
ISQLQuery sqlQuery = CurrentSession.CreateSQLQuery(myQueryString);//create query
sqlQuery.AddEntity(typeof (T)); //set result type
IQuery query = sqlQuery.SetReadOnly(true);
If you have id's, then you should select them in your query, and compare it to the corresponding id column in the second query.
var elementIdsToUseInContain = query
.Select(x => x.YourIdProperty);
var result = queryable
.Where(x => elementIdsToUseInContain.Contains(x.YourIdPropertyToCompareTo))
.ToList();
Note: I haven't tested it, but it is fairly standard Linq. It depends on if NHibernate supports it, which i do not know.
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))
I am working on a system that the client decided to use status for the records. One of them is X for excluded. What I want to know is if it is possible to run linq queries that adds something like
where status != 'X'
Automatically to not show "excluded" records. Thanks !
Sort of. Queries in Linq are lazily-evaluated, so you can append conditions to them as you like before actually fetching the first result and it'll still result in "optimal" SQL being used.
For example:
// an extension method on the LINQ context:
public static IQueryable<Story> FilteredStories(this DbContext db)
{
return from story in db.Stories where status != "X" select story;
}
// ...later...
var stories = from story in db.FilteredStories()
where title = "Something"
select story;
foreach(var story in stories)
{
// whatever...
}
You could also "hide" the underlying LINQ context and always go through a wrapper class that appends the status != "X" condition. Of course, then problem with that is then you'd have to jump through hoops if you didn't want a filtered list...
What you might be after is Dynamic LINQ. It's a helper library that you can use to parse LINQ expressions from strings.
LINQ is so easy that writing the question practically gives you the answer!
where status != "X"
My preference would be
.Where(record => record.status != "X")
which does exactly the same thing. The only thing you missed is double quotes because it's a string, not a char.
Could someone help me with this exception? I don't understand what it means or how to fix it... It is an SqlException with the following message:
All queries combined using a UNION, INTERSECT or EXCEPT operator must have an equal number of expressions in their target lists.
I get it when running a query in pseudo code looking like this:
// Some filtering of data
var query = data.Subjects
.Where(has value)
.Where(has other value among some set of values);
// More filtering, where I need to have two different options
var a = query
.Where(some foreign key is null);
var b = query
.Where(some foreign key is not null)
.Where(and that foreign key has a property which is what I want);
query = a.Union(b);
// Final filter and then get result as a list
var list = query
.Where(last requirement)
.ToList();
If I remove the a.Union(b) parts, it runs without the exception. So I know the error is there. But why do I get it? And how can I fix it? Am I doing something too crazy here? Have I misunderstood how to use the Union thing?
Basically what I have is some entities which have a foreign key to some other entity. And I need to get all the entities which either have that foreign key set to null or where that foreign entity fulfills some requirements.
Judging from the SQL error you listed you may be experiencing the same issue I was. Basically when Linq to SQL queries that use the Concat or Union extension method on two different queries it appears that there is a bug in Linq to SQL which optimizes each projection separately without regard to the fact that the projection must stay the same in order to accomplish the SQL Union.
References:
LINQ to SQL produces incorrect TSQL when using UNION or CONCAT
Linq to SQL Union Same Fieldname generating Error
If this happens to be your problem as well I've found a solution that is working for me as shown below.
var queryA =
from a in context.TableA
select new
{
id,
name,
onlyInTableA,
}
var queryB =
from b in context.TableB
let onlyInTableA = default(string)
select new
{
id,
name,
onlyInTableA,
}
var results = queryA.Union(queryB).ToList();
Since this looks like a problem with the generated SQL, you should try to use either an SQL Profiler, or use this code for DebuggerWritter class to write the SQL to your Output Window in Visual Studio.
The SQL error is normally caused by the fields retrieved for UNION is not the same for the 2 queries. For example, if the first query might have 3 fields, but the second query has 4 fields, this error will occur. So, seeing the generated SQL will definitely help in this case.
Can you perhaps write it in a single query?
.Where(row => row.ForeignKey == null || row.ForeignKey.SomeCondition);
There are also ways of merging expressions (OrElse), but that isn't trivial.
Not sure where the error comes from, though!
edit: haven't tested it, but this should be logically equivalent to a UNION:
public static IQueryable<T> WhereAnyOf<T>(
this IQueryable<T> source,
params Expression<Func<T, bool>>[] predicates)
{
if (source == null) throw new ArgumentNullException("source");
if (predicates == null) throw new ArgumentNullException("predicates");
if (predicates.Length == 0) return source.Where(row => false);
if (predicates.Length == 1) return source.Where(predicates[0]);
var param = Expression.Parameter(typeof(T), "row");
Expression body = Expression.Invoke(predicates[0], param);
for (int i = 1; i < predicates.Length; i++)
{
body = Expression.OrElse(body,
Expression.Invoke(predicates[i], param));
}
return source.Where(Expression.Lambda<Func<T, bool>>(body, param));
}
query = a.Union(b);
Not a good idea to mutate captured variables... Likely the cause of the error.
UPDATE: ok not
Here is another idea. The hint is in the error message.
var a = query
.Where(some foreign key is null)
.Select(x => x);
Or play by adding another 'fake' Where till they do become equal :)
I would call data.GetCommand(query) and analyze the resulting DbCommand (especially the generated SQL string). That should give you a clue to what goes wrong.
There is no projection going on anywhere so I would expect both target lists to be the same.
You could try to reduce your query to a smaller one that still doesn't work. Start with query.Union(query) (this should at least work). Than add your Where calls one by one to see when it stops working.
It must be one of your Where calls that adds extra columns to your select list.
Are you by any chance passing in a value to the 'select' side in a variable, or are you returning the same field more than once? SP1 introduced a bug where it tries to 'optimize' out such things and that can cause union queries to break (due to the query parts 'optimizing' out different passed-in params).
If you post your actual query rather than pseudo code it makes it easier to identify if this is the case.
(And a workaround if this is the case is to materialize the individual parts first and then do a client-side (L2O) union).
jpierson has the problem summarised correctly.
I also had the problem, this time caused by some literals in the select statement:
Dim results = (From t in TestDataContext.Table1 _
Where t.ID = WantedID _
Select t.name, SpecialField = 0, AnotherSpecialField = 0, t.Address).Union _
From t in TestDataContext.Table1 _
Where t.SecondID = WantedSecondID _
Select t.name, SpecialField = 1, AnotherSpecialField = 0, t.Address)
The first sub-query of "SpecialField = 0" and the "AnotherSpecialField = 0" were optimised, resulting in one field instead of two being used in the union, which will obviously fail.
I had to change the first query so that the SpecialField & AnotherSpecialField had different values, much like in the second sub-query.
Well I had an issue with this. Using Sql 08 i had two table functions that returned an int and a string in both cases. I created a complex object and used linq to attempt a UNION. Had an IEqualityComparer to do the comparision. All compiled fine, but crashed with a unsupported overload. Ok, i realised the problem discussed seemed to smack of defered execution. So i get the collections, and place ToList(), then do the UNION and it is all good. Not sure if this is helpful, but it works for me