Consider the following Query :
var profilelst =
(
from i in dbContext.ProspectProfiles
where i.CreateId == currentUser
select new ProspectProfile
{
ProspectId = i.ProspectId,
Live = i.Live,
Name = i.Name,
ServiceETA = i.Opportunities.OrderByDescending(t => t.FollowUpDate)
.FirstOrDefault()
.ServiceETA.ToString(),
FollowUpDate = i.Opportunities.OrderByDescending(t => t.FollowUpDate)
.FirstOrDefault()
.FollowUpDate
}
)
.ToList();
return profilelst.OrderByDescending(c=>c.FollowUpDate)
.Skip(0).Take(endIndex)
.ToList();
Here in this query please take a look at FollowUpDate and ServiceType, these both i have fetched from Opportunity table, is there any other work around to get these both..
One to Many Relationship in tables is like: ProspectProfile -> Opportunities
Whether the query i have written is ok or is there any another work around that can be done in easier way.
The only thing you can improve is to avoid ordering twice by changing your code to this:
var profilelst
= dbContext.ProspectProfiles
.Where(i => i.CreateId == currentUser)
.Select(i =>
{
var opportunity
= i.Opportunities
.OrderByDescending(t => t.FollowUpDate)
.First();
return new ProspectProfile
{
ProspectId = i.ProspectId,
Live = i.Live,
Name = i.Name,
ServiceETA = opportunity.ServiceETA.ToString(),
FollowUpDate = opportunity.FollowUpDate
}
}).ToList();
return profilelst.OrderByDescending(c => c.FollowUpDate).Take(endIndex).ToList();
I made several changes to your original query:
I changed it to use method chains syntax. It is just so much easier to read in my opinion.
I removed the unnecessary Skip(0).
The biggest change is in the Select part:
I changed FirstOrDefault to First, because you are accessing the properties of the return value anyway. This will throw a descriptive exception if no opportunity exists. That's better than what you had: In your case it would throw a NullReferenceException. That's bad, NullReferenceExceptions always indicate a bug in your program and are not descriptive at all.
I moved the part that selects the opportunity out of the initializer, so we need to do the sorting only once instead of twice.
There are quite a few problems in your query:
You cannot project into an entity (select new ProspectProfile). LINQ to Entities only supports projections into anonymous types (select new) or other types which are not part of your entity data model (select new MySpecialType)
ToString() for a numeric or DateTime type is not supported in LINQ to Entities (ServiceETA.ToString())
FirstOrDefault().ServiceETA (or FollowUpdate) will throw an exception if the Opportunities collection is empty and ServiceETA is a non-nullable value type (such as DateTime) because EF cannot materialize any value into such a variable.
Using .ToList() after your first query will execute the query in the database and load the full result. Your later Take happens in memory on the full list, not in the database. (You effectively load the whole result list from the database into memory and then throw away all objects except the first you have Takeen.
To resolve all four problems you can try the following:
var profilelst = dbContext.ProspectProfiles
.Where(p => p.CreateId == currentUser)
.Select(p => new
{
ProspectId = p.ProspectId,
Live = p.Live,
Name = p.Name,
LastOpportunity = p.Opportunities
.OrderByDescending(o => o.FollowUpDate)
.Select(o => new
{
ServiceETA = o.ServiceETA,
FollowUpDate = o.FollowUpDate
})
.FirstOrDefault()
})
.OrderByDescending(x => x.LastOpportunity.FollowUpDate)
.Skip(startIndex) // can be removed if startIndex is 0
.Take(endIndex)
.ToList();
This will give you a list of anonymous objects. If you need the result in a list of your entity ProspectProfile you must copy the values after this query. Note that LastOpportunity can be null in the result if a ProspectProfile has no Opportunities.
Related
Novice C#, veteran DBA and brand new to LINQ. I'm trying to understand why a select clause, within context of a groupBy, is returning a type name (in this case "System.Linq.Enumerable+WhereSelectEnumerableIterator`2[CasesReportParser.Requisition,System.String]") rather than a field value.
My ultimate confusion, I believe, is generically how to include additional fields in a groupBy result with those additional fields not being grouped on (not otherwise available in g.Key).
Here is the code and it is the conditional, ExamType, on the last line that is returning the type name. Thank you.
var ListFinal = rows
.GroupBy(l => new { ExamDate = ParseDate(l.ExamDate), l.PatientID, l.PatientFirst, l.PatientLast, l.Birthdate, l.SiteName })
.Select(g => new {
//ExamDate = g.Select(l => l.ExamDate),//.ToString().Substring(0,10), //.ToString().Split(' ')[0]
ExamDate = g.Key.ExamDate,
PatientID = g.Key.PatientID,
PatientFirst = g.Key.PatientFirst,
PatientLast = g.Key.PatientLast,
Birthdate = g.Key.Birthdate,
SiteName = g.Key.SiteName,
ReqCount = g.Select(l => l.RequisitionNumber).Distinct().Count(),
ExamCount = g.Select(l => l.ExamID).Distinct().Count(),
ExamType = (g.Select(l => l.ExamID).Distinct().Count()>1 ? "multiple" : g.Select(l => l.ExamType).ToString())
});
You might want to add .First, because it is returning a collection.
g.Select(l => l.ExamType).First().ToString()
First of Select doesn´t return any elements at all. In fact it does not even return a collection, but an iterator that can iterate over a collection.
So just calling Select on a collection won´t do anything at all, only calling MoveNext on that iterator will perform the query. This can be done in many difefrent ways, one of which is by calling First or FirstOrDefault, which seems what you should use in your case. First will throw an exception if no element was returned, FirstOrDefault however will return the default-value for the type of the iterator (null for reference-types).
Having said this you probably need this in your Select:
ExamType = g.Select(l => l.ExamID).Distinct().Count() > 1 ?
"multiple" :
g.FirstOrDefault(l => l.ExamType)?.ToString()
As FirstOrDefault may return null you´d get a NullReferenceException, which is why you should use the ?.-operator.
//This works, but seems incorrect to me
Object selection = db.ExampleTable
.Where(s => s.Id == id)
.Select(s => new { s.Id, s.PropIWantToShow })
.SingleOrDefault();
//This seems correct, but does not work
Object selection = db.ExampleTable
.SingleOrDefault(s => s.Id == id)
.Select(s => new { s.Id, s.PropIWantToShow });
db is our Entity Framework data context.
My goal is to select a single entry matching the provided id in ExampleTable. If an entry is not found, this is to return null. However, EF doesn't seem to let me select a single object and then only return specific properties. How do I accomplish this or is the first example I provided correct?
I did check this question:
select properties of entity ef linq:
Unfortunately you cannot conditionally load properties of related entity - you either load whole door entity, or don't include that entity.
But the answer just doesn't seem right, but obviously "seems" is a very weak statement.
Your first method is correct:
//This works, but seems incorrect to me
Object selection = db.ExampleTable
.Where(s => s.Id == id)
.Select(s => new { s.Id, s.PropIWantToShow })
.SingleOrDefault();
Your second method gets you a single object, not an IQueryable<T> object that LINQ would work with. If you want to convert from one type of object to another, that isn't a LINQ thing. You can still, but it'll be more convoluted. Something like:
var selection =...;
var newselection=new { Id=selection.Id, PropIWantToShow=selection.PropIWantToShow };
but this is very bad because you DID retrieve the entire object from the DB, and then just threw away most of it. Your first method only returns 2 fields from the DB.
If you want your function to return null if condition doesn't match then use FirstorDefault() instead of SingleorDefalut(). So if you want to match an id and return an object then do it like this :
return db.ExampleTable.FirstorDefault(c=>c.Id == id);
I have a LINQ query with a sublist in it which sometimes can return no returns, however can't manage to get the actual null instead of a error.
Any help in this would be appriciated.
var member_settings = from ml in _pMetaLanguages
join s in _settings
on ml.id equals s.setting_type_id
orderby ml.name descending
select new _class {
group_name = ml.name
, code = (ml.classes.Where(c => c.id == s.int_value)).FirstOrDefault().code
, name = (ml.classes.Where(c => c.id == s.int_value)).FirstOrDefault().name
, id = (ml.classes.Where(c => c.id == s.int_value)).FirstOrDefault().id
,
classUI = ml.metadataMUI
,
nameUI = ml.metadataMUI.Where(u => u.language_type_id.Equals(list_language_id))
.Select(i => new pMetaClasses
{
name = i.classes.Where(y => (y.bound_id.Equals(list_language_id))).FirstOrDefault().name
}).FirstOrDefault().name
, setting_type_id = s.setting_type_id
, int_value = s.int_value
};
EDITED
This part delivers the problems
nameUI = ml.metadataMUI.Where(u => u.language_type_id.Equals(list_language_id))
.Select(i => new pMetaClasses
{
name = i.classes.Where(y => (y.bound_id.Equals(list_language_id))).FirstOrDefault().name
}).FirstOrDefault().name
"metadataMUI" sometimes has records but not always, when no records it should be null (at least no error...
You have a collection of objects, it may or may not be empty. You want to pull out a property from the first item in that collection, if there is one. Currently whenever your query is face with this problem you are using this general approach:
sequence.FirstOrDefault().SomeMember
This code works fine if the sequence will always contain an item. It doesn't work if it may be empty. (Unless you can and want to get the member of the default value. For reference types, as you clearly are working with, this just results in null reference excpetions.) Fortunately there is a simple transformation to ensure that this works correctly. Simply use Select to transform the sequence into the sub-property that you are interested in and then get the first or default value of that sequence:
sequence.Select(item => item.SomeMember).FirstOrDefault();
This will work properly whether there are items or no items. The projection will never be called on default values using this approach.
Of course you do this in a lot of places, so you'll need to make this transformation throughout your query.
I'm re-writing some of my old NHibernate code to be more database agnostic and use NHibernate queries rather than hard coded SELECT statements or database views. I'm stuck with one that's incredibly slow after being re-written. The SQL query is as such:
SELECT
r.recipeingredientid AS id,
r.ingredientid,
r.recipeid,
r.qty,
r.unit,
i.conversiontype,
i.unitweight,
f.unittype,
f.formamount,
f.formunit
FROM recipeingredients r
INNER JOIN shoppingingredients i USING (ingredientid)
LEFT JOIN ingredientforms f USING (ingredientformid)
So, it's a pretty basic query with a couple JOINs that selects a few columns from each table. This query happens to return about 400,000 rows and has roughly a 5 second execution time. My first attempt to express it as an NHibernate query was as such:
var timer = new System.Diagnostics.Stopwatch();
timer.Start();
var recIngs = session.QueryOver<Models.RecipeIngredients>()
.Fetch(prop => prop.Ingredient).Eager()
.Fetch(prop => prop.IngredientForm).Eager()
.List();
timer.Stop();
This code works and generates the desired SQL, however it takes 120,264ms to run. After that, I loop through recIngs and populate a List<T> collection, which takes under a second. So, something NHibernate is doing is extremely slow! I have a feeling this is simply the overhead of constructing instances of my model classes for each row. However, in my case, I'm only using a couple properties from each table, so maybe I can optimize this.
The first thing I tried was this:
IngredientForms joinForm = null;
Ingredients joinIng = null;
var recIngs = session.QueryOver<Models.RecipeIngredients>()
.JoinAlias(r => r.IngredientForm, () => joinForm)
.JoinAlias(r => r.Ingredient, () => joinIng)
.Select(r => joinForm.FormDisplayName)
.List<String>();
Here, I just grab a single value from one of my JOIN'ed tables. The SQL code is once again correct and this time it only grabs the FormDisplayName column in the select clause. This call takes 2498ms to run. I think we're on to something!!
However, I of course need to return several different columns, not just one. Here's where things get tricky. My first attempt is an anonymous type:
.Select(r => new { DisplayName = joinForm.FormDisplayName, IngName = joinIng.DisplayName })
Ideally, this should return a collection of anonymous types with both a DisplayName and an IngName property. However, this causes an exception in NHibernate:
Object reference not set to an instance of an object.
Plus, .List() is trying to return a list of RecipeIngredients, not anonymous types. I also tried .List<Object>() to no avail. Hmm. Well, perhaps I can create a new type and return a collection of those:
.Select(r => new TestType(r))
The TestType construction would take a RecipeIngredients object and do whatever. However, when I do this, NHibernate throws the following exception:
An unhandled exception of type 'NHibernate.MappingException' occurred
in NHibernate.dll
Additional information: No persister for: KitchenPC.Modeler.TestType
I guess NHibernate wants to generate a model matching the schema of RecipeIngredients.
How can I do what I'm trying to do? It seems that .Select() can only be used for selecting a list of a single column. Is there a way to use it to select multiple columns?
Perhaps one way would be to create a model with my exact schema, however I think that would end up being just as slow as the original attempt.
Is there any way to return this much data from the server without the massive overhead, without hard coding a SQL string into the program or depending on a VIEW in the database? I'd like to keep my code completely database agnostic. Thanks!
The QueryOver syntax for conversion of selected columns into artificial object (DTO) is a bit different. See here:
16.6. Projections for more details and nice example.
A draft of it could be like this, first the DTO
public class TestTypeDTO // the DTO
{
public string PropertyStr1 { get; set; }
...
public int PropertyNum1 { get; set; }
...
}
And this is an example of the usage
// DTO marker
TestTypeDTO dto = null;
// the query you need
var recIngs = session.QueryOver<Models.RecipeIngredients>()
.JoinAlias(r => r.IngredientForm, () => joinForm)
.JoinAlias(r => r.Ingredient, () => joinIng)
// place for projections
.SelectList(list => list
// this set is an example of string and int
.Select(x => joinForm.FormDisplayName)
.WithAlias(() => dto.PropertyStr1) // this WithAlias is essential
.Select(x => joinIng.Weight) // it will help the below transformer
.WithAlias(() => dto.PropertyNum1)) // with conversion
...
.TransformUsing(Transformers.AliasToBean<TestTypeDTO>())
.List<TestTypeDTO>();
So, I came up with my own solution that's a bit of a mix between Radim's solution (using the AliasToBean transformer with a DTO, and Jake's solution involving selecting raw properties and converting each row to a list of object[] tuples.
My code is as follows:
var recIngs = session.QueryOver<Models.RecipeIngredients>()
.JoinAlias(r => r.IngredientForm, () => joinForm)
.JoinAlias(r => r.Ingredient, () => joinIng)
.Select(
p => joinIng.IngredientId,
p => p.Recipe.RecipeId,
p => p.Qty,
p => p.Unit,
p => joinIng.ConversionType,
p => joinIng.UnitWeight,
p => joinForm.UnitType,
p => joinForm.FormAmount,
p => joinForm.FormUnit)
.TransformUsing(IngredientGraphTransformer.Create())
.List<IngredientBinding>();
I then implemented a new class called IngredientGraphTransformer which can convert that object[] array into a list of IngredientBinding objects, which is what I was ultimately doing with this list anyway. This is exactly how AliasToBeanTransformer is implemented, only it initializes a DTO based on a list of aliases.
public class IngredientGraphTransformer : IResultTransformer
{
public static IngredientGraphTransformer Create()
{
return new IngredientGraphTransformer();
}
IngredientGraphTransformer()
{
}
public IList TransformList(IList collection)
{
return collection;
}
public object TransformTuple(object[] tuple, string[] aliases)
{
Guid ingId = (Guid)tuple[0];
Guid recipeId = (Guid)tuple[1];
Single? qty = (Single?)tuple[2];
Units usageUnit = (Units)tuple[3];
UnitType convType = (UnitType)tuple[4];
Int32 unitWeight = (int)tuple[5];
Units rawUnit = Unit.GetDefaultUnitType(convType);
// Do a bunch of logic based on the data above
return new IngredientBinding
{
RecipeId = recipeId,
IngredientId = ingId,
Qty = qty,
Unit = rawUnit
};
}
}
Note, this is not as fast as doing a raw SQL query and looping through the results with an IDataReader, however it's much faster than joining in all the various models and building the full set of data.
IngredientForms joinForm = null;
Ingredients joinIng = null;
var recIngs = session.QueryOver<Models.RecipeIngredients>()
.JoinAlias(r => r.IngredientForm, () => joinForm)
.JoinAlias(r => r.Ingredient, () => joinIng)
.Select(r => r.column1, r => r.column2})
.List<object[]>();
Would this work?
I have got a bit of an issue and was wondering if there is a way to have my cake and eat it.
Currently I have a Repository and Query style pattern for how I am using Linq2Sql, however I have got one issue and I cannot see a nice way to solve it. Here is an example of the problem:
var someDataMapper = new SomeDataMapper();
var someDataQuery = new GetSomeDataQuery();
var results = SomeRepository.HybridQuery(someDataQuery)
.Where(x => x.SomeColumn == 1 || x.SomeColumn == 2)
.OrderByDescending(x => x.SomeOtherColumn)
.Select(x => someDataMapper.Map(x));
return results.Where(x => x.SomeMappedColumn == "SomeType");
The main bits to pay attention to here are Mapper, Query, Repository and then the final where clause. I am doing this as part of a larger refactor, and we found that there were ALOT of similar queries which were getting slightly different result sets back but then mapping them the same way to a domain specific model. So take for example getting back a tbl_car and then mapping it to a Car object. So a mapper basically takes one type and spits out another, so exactly the same as what would normally happen in the select:
// Non mapped version
select(x => new Car
{
Id = x.Id,
Name = x.Name,
Owner = x.FirstName + x.Surname
});
// Mapped version
select(x => carMapper.Map(x));
So the car mapper is more re-usable on all areas which do similar queries returning same end results but doing different bits along the way. However I keep getting the error saying that Map is not able to be converted to SQL, which is fine as I dont want it to be, however I understand that as it is in an expression tree it would try to convert it.
{"Method 'SomeData Map(SomeTable)' has no supported translation to SQL."}
Finally the object that is returned and mapped is passed further up the stack for other objects to use, which make use of Linq to SQL's composition abilities to add additional criteria to the query then finally ToList() or itterate on the data returned, however they filter based on the mapped model, not the original table model, which I believe is perfectly fine as answered in a previous question:
Linq2Sql point of retrieving data
So to sum it up, can I use my mapping pattern as shown without it trying to convert that single part to SQL?
Yes, you can. Put AsEnumerable() before the last Select:
var results = SomeRepository.HybridQuery(someDataQuery)
.Where(x => x.SomeColumn == 1 || x.SomeColumn == 2)
.OrderByDescending(x => x.SomeOtherColumn)
.AsEnumerable()
.Select(x => someDataMapper.Map(x));
Please note, however, that the second Where - the one that operates on SomeMappedColumn - will now be executed in memory and not by the database. If this last where clause significantly reduces the result set this could be a problem.
An alternate approach would be to create a method that returns the expression tree of that mapping. Something like the following should work, as long as everything happening in the mapping is convertible to SQL.
Expression<Func<EntityType, Car>> GetCarMappingExpression()
{
return new Expression<Func<EntityType, Car>>(x => new Car
{
Id = x.Id,
Name = x.Name,
Owner = x.FirstName + x.Surname
});
}
Usage would be like this:
var results = SomeRepository.HybridQuery(someDataQuery)
.Where(x => x.SomeColumn == 1 || x.SomeColumn == 2)
.OrderByDescending(x => x.SomeOtherColumn)
.Select(GetCarMappingExpression());