Here is a foreach statement I'd like to express in Linq to Entities. It loops through child entities (attachments) of a parent entity (currentFactSheet) to see if an attachment exists with a particular FileName. How can I condense this procedural code into a Linq to Entites statement?
FactSheet currentFactSheet = mainWindow.GetCurrentFactSheet();
bool attachmentExists = false;
foreach (var thisAttachment in currentFactSheet.AttachmentsNav)
{
if (thisAttachment.FileName == nameOfAttachedFile)
{
attachmentExists = true;
}
}
This is a partial image showing FactSheet (left) and the Attachment entity associated via a navigation property named AttachmentsNav:
I want to query in memory entities to avoid a round trip to the database. I've found examples like this that search only the parent level. I've made many attempts, but they never bring up intellisense with the field names on my child entity (specifically Attachment.FileName).
Thanks in advance.
Try this:
bool attachmentExists = currentFactSheet.AttachmentsNav.Any(a => a.FileName == nameOfAttachedFile);
Related
Thanks for checking this out.
My situation is that I have a system where the user can create custom filtered views which I build into a linq query on the request. On the interface they want to see the counts of all the views they have created; pretty straight forward. I'm familiar with combining multiple queries into a single call but in this case I don't know how many queries I have initially.
Does anyone know of a technique where this loop combines the count queries into a single query that I can then execute with a ToList() or FirstOrDefault()?
//TODO Performance this isn't good...
foreach (IMeetingViewDetail view in currentViews)
{
view.RecordCount = GetViewSpecificQuery(view.CustomFilters).Count();
}
Here is an example of multiple queries combined as I'm referring to. This is two queries which I then combine into an anonymous projection resulting in a single request to the sql server.
IQueryable<EventType> eventTypes = _eventTypeService.GetRecords().AreActive<EventType>();
IQueryable<EventPreferredSetup> preferredSetupTypes = _eventPreferredSetupService.GetRecords().AreActive<EventPreferredSetup>();
var options = someBaseQuery.Select(x => new
{
EventTypes = eventTypes.AsEnumerable(),
PreferredSetupTypes = preferredSetupTypes.AsEnumerable()
}).FirstOrDefault();
Well, for performance considerations, I would change the interface from IEnumerable<T> to a collection that has a Count property. Both IList<T> and ICollection<T> have a count property.
This way, the collection object is keeping track of its size and you just need to read it.
If you really wanted to avoid the loop, you could redefine the RecordCount to be a lazy loaded integer that calls GetViewSpecificQuery to get the count once.
private int? _recordCount = null;
public int RecordCount
{
get
{
if (_recordCount == null)
_recordCount = GetViewSpecificQuery(view.CustomFilters).Count;
return _recordCount.Value;
}
}
I have an Abstract type called Product, and five "Types" that inherit from Product in a table per type hierarchy fashion as below:
I want to get all of the information for all of the Products, including a smattering of properties from the different objects that inherit from products to project them into a new class for use in an MVC web page. My linq query is below:
//Return the required products
var model = from p in Product.Products
where p.archive == false && ((Prod_ID == 0) || (p.ID == Prod_ID))
select new SearchViewModel
{
ID = p.ID,
lend_name = p.Lender.lend_name,
pDes_rate = p.pDes_rate,
pDes_details = p.pDes_details,
pDes_totTerm = p.pDes_totTerm,
pDes_APR = p.pDes_APR,
pDes_revDesc = p.pDes_revDesc,
pMax_desc = p.pMax_desc,
dDipNeeded = p.dDipNeeded,
dAppNeeded = p.dAppNeeded,
CalcFields = new DAL.SearchCalcFields
{
pDes_type = p.pDes_type,
pDes_rate = p.pDes_rate,
pTFi_fixedRate = p.pTFi_fixedRate
}
}
The problem I have is accessing the p.pTFi_fixedRate, this is not returned with the Products collection of entities as it is in the super type of Fixed. How do I return the "super" type of Products (Fixed) properties using Linq and the Entity Framework. I actually need to return some fields from all the different supertypes (Disc, Track, etc) for use in calculations. Should I return these as separate Linq queries checking the type of "Product" that is returned?
This is a really good question. I've had a look in the Julie Lerman book and scouted around the internet and I can't see an elegant answer.
If it were me I would create a data transfer object will all the properties of the types and then have a separate query for each type and then union them all up. I would insert blanks into the DTO properies where the properties aren't relevant to that type. Then I would hope that the EF engine makes a reasonable stab at creating decent SQL.
Example
var results = (from p in context.Products.OfType<Disc>
select new ProductDTO {basefield1 = p.val1, discField=p.val2, fixedField=""})
.Union(
from p in context.Products.OfType<Fixed>
select new ProductDTO {basefield1 = p.val1, discField="", fixedField=p.val2});
But that can't be the best answer can it. Is there any others?
So Fixed is inherited from Product? If so, you should probably be querying for Fixed instead, and the Product properties will be pulled into it.
If you are just doing calculations and getting some totals or something, you might want to look at using a stored procedure. It will amount to fewer database calls and allow for much faster execution.
Well it depends on your model, but usually you need to do something like:
var model = from p in Product.Products.Include("SomeNavProperty")
.... (rest of query)
Where SomeNavProperty is the entity type that loads pTFi_fixedRate.
I am using Entity Framework for the first time and noticed that the entities object returns entity collections.
DBEntities db = new DBEntities();
db.Users; //Users is an ObjectSet<User>
User user = db.Users.Where(x => x.Username == "test").First(); //Is this getting executed in the SQL or in memory?
user.Posts; //Posts is an EntityCollection<Post>
Post post = user.Posts.Where(x => x.PostID == "123").First(); //Is this getting executed in the SQL or in memory?
Do both ObjectSet and EntityCollection implement IQueryable? I am hoping they do so that I know the queries are getting executed at the data source and not in memory.
EDIT: So apparently EntityCollection does not while ObjectSet does. Does that mean I would be better off using this code?
DBEntities db = new DBEntities();
User user = db.Users.Where(x => x.Username == "test").First(); //Is this getting executed in the SQL or in memory?
Post post = db.Posts.Where(x => (x.PostID == "123")&&(x.Username == user.Username)).First(); // Querying the object set instead of the entity collection.
Also, what is the difference between ObjectSet and EntityCollection? Shouldn't they be the same?
Thanks in advance!
EDIT: Sorry, I'm new to this. I'm trying to understand. Attached EntityCollections are lazy loaded, so if I access them then memory is populated with them. Rather than doing two querys to the object sets like in my last edit, I am curious if this query would be more what I was after:
DBEntities db = new DBEntities();
User user = (from x in db.Users
from y in x.Posts
where x.Username == "test"
where y.PostID == 123
select x).First();
ObjectSet<T> does implement IQueryable<T>, but EntityCollection<T> does not.
The difference is that ObjectSet<T> is meant to be used for querying directly (which is why it does implement the interface). EntityCollection<T>, on the other hand, is used for the "many" end of a result set, typically returned in a query done on an ObjectSet<T>. As such, it impelments IEnumerable<T>, but not IQueryable<T> (as it's already the populated results of a query).
I was almost ready to say yes, they both do. Luckily I check the documentation first.
EntityCollection does not implement IQueryable.
As for the difference, ObjectSet<TEntity> represents the the objects generated from a table in a database. EntityCollection<TEntity> represents a collection of entity objects on the 'Many' side of One to Many or Many to Many relationship.
I want to create an Entity Object from a LinQ statement, but I don't want to load all its columns.
My ORDERS object has a lot of columns, but I just want to retrieve the REFERENCE and OPERATION columns so the SQL statement and result will be smaller.
This LinQ statement works properly and loads all my object attributes:
var orders = (from order in context.ORDERS
select order);
However the following statement fails to load only two properties of my object
var orders = (from order in context.ORDERS
select new ORDERS
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION
});
The error thrown is:
The entity or complex type
'ModelContextName.ORDERS' cannot be
constructed in a LINQ to Entities
query.
What is the problem? Isn't it possible to partially load an object this way?
Thank you in advance for your answers.
ANSWER
Ok I should thank you both Yakimych and Dean because I use both of your answers, and now I have:
var orders = (from order in context.ORDERS
select new
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION,
})
.AsEnumerable()
.Select(o =>
(ORDERS)new ORDERS
{
REFERENCE = o.REFERENCE,
OPERATION = o.OPERATION
}
).ToList().AsQueryable();
And I get exactly what I want, the SQL Statement is not perfect but it returns only the 2 columns I need (and another column which contains for every row "1" but I don't know why for the moment) –
I also tried to construct sub objects with this method and it works well.
No, you can't project onto a mapped object. You can use an anonymous type instead:
var orders = (from order in context.ORDERS
select new
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION
});
The problem with the above solution is that from the moment you call AsEnumerable(), the query will get executed on the database. In most of the cases, it will be fine. But if you work with some large database, fetching the whole table(or view) is probably not what you want. So, if we remove the AsEnumerable, we are back to square 1 with the following error:
The entity or complex type 'ModelContextName.ORDERS' cannot be constructed in a LINQ to Entities query.
I have been struggling with this problem for a whole day and here is what I found. I created an empty class inheriting from my entity class and performed the projection using this class.
public sealed class ProjectedORDERS : ORDERS {}
The projected query (using covariance feature):
IQueryable<ORDERS> orders = (from order in context.ORDERS
select new ProjectedORDERS
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION,
});
Voilà! You now have a projected query that will map to an entity and that will get executed only when you want to.
I think the issue is creating new entities within the query itself, so how about trying this:
context.ORDERS.ToList().Select(o => new ORDERS
{
REFERENCE = o.REFERENCE,
OPERATION = o.OPERATION
});
Is there a way to ensure a particular conditional clause is added to the expression tree on each select from a particular table?
For example, a table with a field with the date a record was deleted should never come back or be included in any kind of statement.
Rather than include a where clause each time, is there a way, without creating a view, to add a conditional to each select?
--- Edit for clarity below ---
I'm looking for a function I can partial, much like the Insert/Update/Delete functions, but for Selecting. I want to apply a blanket filter to all queries against a table.
Furthermore, if I get a collection of items from a parent, I want that set to be filtered as well.
Something like:
Private Function BaseItems() As IQueryable(Of Item)
Return (From mi In dataContext.Items Where mi.DeletedAt Is Nothing Select mi)
End Function
Public Function GetItems() as list(of Item)
Return (From mi in BaseItems() select mi).ToList()
End Function
works for functions I write and call. ITEMS, can be a child of MASTER, for example.
'assume TheMaster is a LinqToSQL data class which has a one to many child of Items
TheMaster.Items.Count '<-- will bring back all Items.
How do I always filter what populates by data classes?
You should be able to do this my putting the items into a list and then use lambda expressions to filter the list?
MyListObject.Where(x => x == x.Date);
I'm not sure I understand the context of your question. However you can add more where conditionals on a linq query. If you return an IQueryable, you'll only return a runnable query and you can chain on it other LINQ queries. Afaik it won't execute until you start making it to an IEnumerable or List to iterate through.
Example on LINQ-to-SQL:
MyDataContext context = new MyDataContext();
public IQueryable<MyTable> GetTable() {
return from record in context.Records
where record.Date > DateTime.Now
select record;
}
public IEnumerable<MyTable> GetTableWithinWeek() {
return from record in GetTable()
where record.Date < DateTime.Now.AddDays(7);
select record;
}
I hope my answer makes sense.
AssociateWith in the DataLoadOptions of the DataContext seems to do what I'm needing.