Ok, I must be working too hard because I can't get my head around what it takes to use the Entity Framework correctly.
Here is what I am trying to do:
I have two tables: HeaderTable and DetailTable. The DetailTable will have 1 to Many records for each row in HeaderTable. In my EDM I set up a Relationship between these two tables to reflect this.
Since there is now a relationship setup between these tables, I thought that by quering all the records in HeaderTable, I would be able to access the DetailTable collection created by the EDM (I can see the property when quering, but it's null).
Here is my query (this is a Silverlight app, so I am using the DomainContext on the client):
// myContext is instatiated with class scope
EntityQuery<Project> query = _myContext.GetHeadersQuery();
_myContext.Load<Project>(query);
Since these calls are asynchronous, I check the values after the callback has completed. When checking the value of _myContext.HeaderTable I have all the rows expected. However, the DetailsTable property within _myContext.HeaderTable is empty.
foreach (var h in _myContext.HeaderTable) // Has records
{
foreach (var d in h.DetailTable) // No records
{
string test = d.Description;
}
I'm assuming my query to return all HeaderTable objects needs to be modified to somehow return all the HeaderDetail collectoins for each HeaderTable row. I just don't understand how this non-logical modeling stuff works yet.
What am I doing wrong? Any help is greatly appriciated. If you need more information, just let me know. I will be happy to provide anything you need.
Thanks,
-Scott
What you're probably missing is the Include(), which I think is out of scope of the code you provided.
Check out this cool video; it explained everything about EDM and Linq-to-Entities to me:
http://msdn.microsoft.com/en-us/data/ff628210.aspx
In case you can't view video now, check out this piece of code I have based on those videos (sorry it's not in Silverlight, but it's the same basic idea, I hope).
The retrieval:
public List<Story> GetAllStories()
{
return context.Stories.Include("User").Include("StoryComments").Where(s => s.HostID == CurrentHost.ID).ToList();
}
Loading the the data:
private void LoadAllStories()
{
lvwStories.DataSource = TEContext.GetAllStories();
lvwStories.DataBind();
}
Using the data:
protected void lvwStories_ItemDataBound(object sender, ListViewItemEventArgs e)
{
if (e.Item.ItemType == ListViewItemType.DataItem)
{
Story story = e.Item.DataItem as Story;
// blah blah blah....
hlStory.Text = story.Title;
hlStory.NavigateUrl = "StoryView.aspx?id=" + story.ID;
lblStoryCommentCount.Text = "(" + story.StoryComments.Count.ToString() + " comment" + (story.StoryComments.Count > 1 ? "s" : "") + ")";
lblStoryBody.Text = story.Body;
lblStoryUser.Text = story.User.Username;
lblStoryDTS.Text = story.AddedDTS.ToShortTimeString();
}
}
Related
I have a LINQ query
var age = new int[]{1,2,3};
dbContext.TA.WHERE(x=> age.Contains( x.age)).ToList()
In an online article #11 (https://medium.com/swlh/entity-framework-common-performance-mistakes-cdb8861cf0e7) mentioned it is not a good practice as it creates many execution plan at the SQL server.
In this case, how should LINQ be revised so that I can do the same thing but minimize the amount of execution plans generated?
(note that I have no intention to convert it into a stored procedure and pass & join with the UDT as again it requires too many effort to do so)
That article offers some good things to keep in mind when writing expressions for EF. As a general rule that example is something to keep in mind, not a hard "never do this" kind of rule. It is a warning over writing queries that allow for multi-select and to avoid this when possible as it will be on the more expensive side.
In your example with something like "Ages", having a hard-coded list of values does not cause a problem because every execution uses the same list. (until the app is re-compiled with a new list, or you have code that changes the list for some reason.) Examples where it can be perfectly valid to use this is with something like Statuses where you have a status Enum. If there are a small number of valid statuses that a record can have, then declaring a common array of valid statuses to use in an Contains clause is fine:
public void DeleteEnquiry(int enquiryId)
{
var allowedStatuses = new[] { Statuses.Pending, Statuses.InProgress, Statuses.UnderReview };
var enquiry = context.Enquiries
.Where(x => x.EnquiryId == enquiryId && allowedStatuses.Contains(x.Status))
.SingleOrDefault();
try
{
if(enquiry != null)
{
enquiry.IsActive = false;
context.SaveChanges();
}
else
{
// Enquiry not found or invalid status.
}
}
catch (Exception ex) { /* handle exception */ }
}
The statuses in the list aren't going to change so the execution plan is static for that context.
The problem is where you accept something like a parameter with criteria that include a list for a Contains clause.
it is highly unlikely that someone would want to load data where a user could select ages "2, 4, and 6", but rather they would want to select something like: ">=2", or "<=6, or "2>=6" So rather than creating a method that accepts a list of acceptable ages:
public IEnumerable<Children> GetByAges(int[] ages)
{
return _dbContext.Children.Where(x => ages.Contains( x.Age)).ToList();
}
You would probably be better served with ranging the parameters:
private IEnumerable<Children> GetByAgeRange(int? minAge = null, int? maxAge = null)
{
var query = _dbContext.Children.AsQueryable();
if (minAge.HasValue)
query = query.Where(x => x.Age >= minAge.Value);
if (maxAge.HasValue)
query = query.Where(x => x.Age <= maxAge.Value);
return query.ToList();
}
private IEnumerable<Children> GetByAge(int age)
{
return _dbContext.Children.Where(x => x.Age == age).ToList();
}
Our current system is using Lazyloading by default (it is something I am going to be disabling but it can't be done right now)
For this basic query I want to return two tables, CustomerNote and Note.
This is my query
using (var newContext = new Entities(true))
{
newContext.Configuration.LazyLoadingEnabled = false;
var result = from customerNotes in newContext.CustomerNotes.Include(d=>d.Note)
join note in newContext.Notes
on customerNotes.NoteId equals note.Id
where customerNotes.CustomerId == customerId
select customerNotes;
return result.ToList();
}
My result however only contains the data in the CustomerNote table
The linked entities Customer and Note are both null, what am I doing wrong here?
I got it working with the following which is much simpler than what I've found elsewhere
Context.Configuration.LazyLoadingEnabled = false;
var result = Context.CustomerNotes.Where<CustomerNote>(d => d.CustomerId == customerId)
.Include(d=>d.Note)
.Include(d=>d.Note.User);
return result.ToList();
This returns my CustomerNote table, related Notes and related Users from the Notes.
That is callled eager loading you want to achieve.
var customerNotes = newContext.CustomerNotes.Include(t=> t.Node).ToList();
This should work, i don't really understand the keyword syntax.
If the code above doesn't work try this:
var customerNotes = newContext.CustomerNotes.Include(t=> t.Node).Select(t=> new {
Node = t.Node,
Item = t
}).ToList();
A newbie question. I am using EntityFramework 4.0. The backend database has a function that will return a subset of records based on time.
Example of working code is:
var query = from rx in context.GetRxByDate(tencounter,groupid)
select rx;
var result = context.CreateDetachedCopy(query.ToList());
return result;
I need to verify that a record does not exist in the database before inserting a new record. Before performing the "Any" filter, I would like to populate the context.Rxes with a subset of the larger backend database using the above "GetRxByDate()" function.
I do not know how to populate "Rxes" before performing any further filtering since Rxes is defined as
IQueryable<Rx> Rxes
and does not allow "Rxes =.. ". Here is what I have so far:
using (var context = new EnityFramework())
{
if (!context.Rxes.Any(c => c.Cform == rx.Cform ))
{
// Insert new record
Rx r = new Rx();
r.Trx = realtime;
context.Add(r);
context.SaveChanges();
}
}
I am fully prepared to kick myself since I am sure the answer is simple.
All help is appreciated. Thanks.
Edit:
If I do it this way, "Any" seems to return the opposite results of what is expected:
var g = context.GetRxByDate(tencounter, groupid).ToList();
if( g.Any(c => c.Cform == rx.Cform ) {....}
I can't find good documentation on these operators. Can someone provide some examples of use and explain what they do?
Entity SQL's CREATEREF reference: http://msdn.microsoft.com/en-us/library/bb386880(v=VS.90)
It's used to "Fabricates references to an entity in an entityset". You can also find references of REF and DEREF from the link.
For VS 2010, the reference is: http://msdn.microsoft.com/en-us/library/bb386880(v=VS.100)
Sample from MSDN:
In the example below, Orders and BadOrders are both entitysets of type
Order, and Id is assumed to be the single key property of Order. The
example illustrates how we may produce a reference to an entity in
BadOrders. Note that the reference may be dangling. That is, the
reference may not actually identify a specific entity. In those cases,
a DEREF operation on that reference returns a null.
select CreateRef(LOB.BadOrders, row(o.Id))
from LOB.Orders as o
Sample code of using entity framework SQL:
using (EntityConnection conn =
new EntityConnection("name=AdventureWorksEntities"))
{
conn.Open();
// Create a query that takes two parameters.
string esqlQuery =
#"SELECT VALUE Contact FROM AdventureWorksEntities.Contact
AS Contact WHERE Contact.LastName = #ln AND
Contact.FirstName = #fn";
try
{
using (EntityCommand cmd = new EntityCommand(esqlQuery, conn))
{
// Create two parameters and add them to
// the EntityCommand's Parameters collection
EntityParameter param1 = new EntityParameter();
param1.ParameterName = "ln";
param1.Value = "Adams";
EntityParameter param2 = new EntityParameter();
param2.ParameterName = "fn";
param2.Value = "Frances";
cmd.Parameters.Add(param1);
cmd.Parameters.Add(param2);
using (DbDataReader rdr = cmd.ExecuteReader(CommandBehavior.SequentialAccess))
{
// Iterate through the collection of Contact items.
while (rdr.Read())
{
Console.WriteLine(rdr["FirstName"]);
Console.WriteLine(rdr["LastName"]);
}
}
}
}
catch (EntityException ex)
{
Console.WriteLine(ex.ToString());
}
conn.Close();
}
I'm currently inserting/updating fields like this (if there's a better way, please say so - we're always learning)
public void UpdateChallengeAnswers(List<ChallengeAnswerInfo> model, Decimal field_id, Decimal loggedUserId)
{
JK_ChallengeAnswers o;
foreach (ChallengeAnswerInfo a in model)
{
o = this.FindChallengeAnswerById(a.ChallengeAnswerId);
if (o == null) o = new JK_ChallengeAnswers();
o.answer = FilterString(a.Answer);
o.correct = a.Correct;
o.link_text = "";
o.link_url = "";
o.position = FilterInt(a.Position);
o.updated_user = loggedUserId;
o.updated_date = DateTime.UtcNow;
if (o.challenge_id == 0)
{
// New record
o.challenge_id = field_id; // FK
o.created_user = loggedUserId;
o.created_date = DateTime.UtcNow;
db.JK_ChallengeAnswers.AddObject(o);
}
else
{
// Update record
this.Save();
}
}
this.Save(); // Commit changes
}
As you can see there is 2 times this.Save() (witch invokes db.SaveChanges();)
when Adding we place the new object into a Place Holder with the AddObject method, in other words, the new object is not committed right away and we can place as many objects we want.
But when it's an update, I need to Save first before moving on to the next object, is there a method that I can use in order to, let's say:
if (o.challenge_id == 0)
{
// New record
o.challenge_id = field_id;
o.created_user = loggedUserId;
o.created_date = DateTime.UtcNow;
db.JK_ChallengeAnswers.AddObject(o);
}
else
{
// Update record
db.JK_ChallengeAnswers.RetainObject(o);
}
}
this.Save(); // Only save once when all objects are ready to commit
}
So if there are 5 updates, I don't need to save into the database 5 times, but only once at the end.
Thank you.
Well if you have an object which is attached to the graph, if you modify values of this object, then the entity is marked as Modified.
If you simply do .AddObject, then the entity is marked as Added.
Nothing has happened yet - only staging of the graph.
Then, when you execute SaveChanges(), EF will translate the entries in the OSM to relevant store queries.
Your code looks a bit strange. Have you debugged through (and ran a SQL trace) to see what is actually getting executed? Because i can't see why you need that first .Save, because inline with my above points, since your modifying the entities in the first few lines of the method, an UPDATE statement will most likely always get executed, regardless of the ID.
I suggest you refactor your code to handle new/modified in seperate method. (ideally via a Repository)
Taken from Employee Info Starter Kit, you can consider the code snippet as below:
public void UpdateEmployee(Employee updatedEmployee)
{
//attaching and making ready for parsistance
if (updatedEmployee.EntityState == EntityState.Detached)
_DatabaseContext.Employees.Attach(updatedEmployee);
_DatabaseContext.ObjectStateManager.ChangeObjectState(updatedEmployee, System.Data.EntityState.Modified);
_DatabaseContext.SaveChanges();
}