I am using dapper and also dapper.contrib.My question is how can I create an generic pagination class.
Here is the what I have tried so far.
public class GenericRepository<T> :IGenericRepository<T> where T : class
{
public async Task<IEnumerable<T>> GetAllPagedAsync(int limit,int offset)
{
var list = await Connection.GetAllAsync<T>();//but this return IEnumarable
return list;
}
}
What I am thinking is get the T name of the class which is the same as Table name,and write an sql string called sql_statement which is apply pagination.later apply this code.
var list = await Connection.QueryAsync<T>("sql_statement")
Does this make sense? I s there any better way to achive that.
It looks currently as though you are planning to retrieve all of the rows in the table and then select from them the page of data you actually require. It would likely be quicker to just select the page you need straight from the database, unless you do actually need all of the rows for some reason.
Assuming that your table names are always going to match exactly with their respective class/entity names, the following will give you a paged result (using postgres):
public class GenericRepository<T> : IGenericRepository<T> where T : class
{
public async Task<IEnumerable<T>> GetAllPagedAsync(int limit, int offset)
{
var tableName = typeof(T).Name;
// assuming here you want the newest rows first, and column name is "created_date"
// may also wish to specify the exact columns needed, rather than *
var query = "SELECT * FROM #TableName ORDER BY created_date DESC Limit #Limit Offset #Offset";
var results = Connection.QueryAsync<T>(query, new {Limit = limit, Offset = offset});
return results;
}
}
A note regarding this. I am obviously not familiar with the structure or size of your database, however for most general purposes the limit/offset approach to paging shown here will most probably be sufficient. There are however some potential issues you may wish to consider:
When the offset value gets very large performance may suffer.
Paging tables with a high frequency of inserts in this fashion may cause results to be duplicated/ appear on multiple pages as the offset values does not take into account new rows added to the table since the last retrieval.
Whether or not these are likely to cause issues to your particular case, these potential drawbacks, as well as some alternatives solutions are outlined here.
Related
We have controllers that read Entities with certain criteria and return a set of view models containing the data to the view. The view uses a Kendo Grid, but I don't think that makes a particular difference.
In each case, we have a Linq Query that gets the overall collection of entity rows and then a foreach loop that creates a model from each row.
Each entity has certain look ups as follows:
those with a 1:1 relationship, e.g. Assigned to (via a foreign key to a single person)
those with a 1:many relationship e.g. copy parties (to 0:many people - there are not many of these)
counts of other relationships (e.g. the number of linked orders)
any (e.g. whether any history exists)
If we do these in the model creation, the performance is not good as the queries must be run separately for each and every row.
We have also tried using includes to eager load the related entities but once you get more than two, this starts to deteriorate too.
I have seen that compiled queries and LoadProperty may be an option and I am particularly interested in the latter.
It would be great to understand best practice in these situations, so I can direct my investigations.
Thanks,
Chris.
Edit - sample code added. However, I'm looking for best practice.
public JsonResult ReadUsersEvents([DataSourceRequest]DataSourceRequest request, Guid userID)
{
var diaryEventModels = new List<DiaryEventModel>();
var events = UnitOfWork.EventRepository.All().Where(e => e.UserID == userID);
foreach (var eventItem in events)
{
var diaryModel = new DiaryEventModel(eventItem);
diaryEventModels.Add(diaryModel);
}
var result = diaryEventModels.ToDataSourceResult(request);
return Json(result, JsonRequestBehavior.AllowGet);
}
public DiaryEventModel(Event eventItem) {
// Regular property from Entity examples - no issue here as data retrived simply in original query
ID = eventItem.ID;
Start = eventItem.StartDateTime;
End = eventItem.EndDateTime;
EventDescription = eventItem.Description;
// One to one looked up Property example
eventModel.Creator = eventItem.Location.FullName;
// Calculation example based on 0 to many looked up properties, also use .Any in some cases
// This is a simplified example
eventModel.AttendeeCount = eventItem.EventAttendees.Count();
// 0 to Many looked up properties
EventAttendees = eventItem.EventAttendees.Select(e => new SelectListItem
{
Text = e.Person.FullName,
Value = e.Person.ID.ToString()
}).ToList();
}
I have a Users table of 76 users and UserGroups table.
Using MVC, OData, a generic repository and EF, I am trying to optimize data retrieval when filtering based on the user group:
/api/Users?$filter=USERGROUPS/any(usergroup: usergroup/ID eq 'Group1')
On the client side, I get the right number of users - 71 (as OData is filtering based on the result), however I want to limit the number of records being returned form the actual query - ie. I do not want to return all records then filter (not optimal for very large data sets).
My API controller method is as follows:
[Queryable(AllowedQueryOptions = AllowedQueryOptions.All)]
public IQueryable<USER> Get()
{
var unitOfWork = new ATMS.Repository.UnitOfWork(_dbContext);
var users = unitOfWork.Repository<USER>()
.Query()
.Include(u => u.USERGROUPS)
.Get()
.OrderBy(order => order.USERNAME);
unitOfWork.Save(); // includes Dispose()
return users.AsQueryable();
}
I read in this post that:
Entity framework takes care of building dynamic query based on the
request.
However, using a SQL Server profiler, the query executed is requesting all the records, rather than a filtered query.
Adding a .Take() to the query does not accomplish the desired result, as we also need the actual number of records returned for paging purposes.
I was thinking of using the grabbing some properties through ODataQueryOptions, but that doesn't seem quite right either.
Is my implementation of Unit of Work and Repository incorrect, in relation to what I am trying to accomplish, and if so, how can this be corrected?
Simple - Just set the Page size for the Queryable atrribute [Queryable(PageSize=10)]
http://www.asp.net/web-api/overview/odata-support-in-aspnet-web-api/supporting-odata-query-options#server-paging
If You'd tell the EF where to apply the options, it would work.
Like this :
//[Queryable(AllowedQueryOptions = AllowedQueryOptions.All)]
public IQueryable<USER> Get(ODataQueryOptions<USER> options)
{
var users = options.ApplyTo(_dbContext.Set<USER>()
.Query()
.Include(u => u.USERGROUPS)
.Get()
.OrderBy(order => order.USERNAME));
return users;
}
Your code didn't work, because it tried to apply the options onto the last line "users.AsQueryable()", so what actually happened, is that EF pull the FULL dataset, and then applied the query onto the last line (that being a in memory collection). And that's why You didn't see that "filter" not being passed to the SQL.
The mechanics are such, that EF tries to apply the Query, to the IQueryable collection that it finds in the code (there's still a question how does it find the correct line).
Bottom Line Up Front
Is there a succinct way that I can constrain the number of child entities that can belong to a parent in Entity Framework. I am using 4.3.1 at the moment.
The Problem
I am developing an ASP.NET MVC3 site which accesses data via a data access layer that uses Entity Framework. I have a SearchList entity which has a many to many relationship to a Search entity. A SearchList may have many Searches, and a Search may belong to many SearchLists.
At one point in the workflow of the site, a user needs to select the searches and other items to use in a batch search. We want the page to load the entire search list.
SearchLists can get quite large, and as a test we created one with 21,000 searches. It took a few seconds, and the data returned was about 9.5 MB, which we were expecting, but jQueryUI choked when trying to table-ify that much.
What we would like
So we want to impose a limit on the number of searches any search list can have. I can go through the application and put a bunch of rules in that checks the size of the collection and if the searches that are trying to be added plus the size of the current... yada yada yada.
If however there was a better way (especially one that could easily output an error message that MVC would pick up) I would totally take that instead.
I have googled, and perused a number of EF blogs to no avail. Constrain children and max number of children in collection and similar searches have returned results that are about Linq queries and the Count and Max methods.
Any help would be appreciated.
There is no built-in way so you will have to code such validation yourselves. Some quick ideas:
You can for example use custom collection for the navigation property which will fire exception when you try to add additional search exceeding the threshold. It is simple but it demands you to have all searches loaded, it will have concurrency problems and moreover it can fire during loading search list and searches from database.
You can handle it in overriden SaveChanges. You will at least have to check how many searches are already related to search list but you will still have concurrency problem (what if other request tries to add search to the same list but only one place is remaining - both can succeed the check and insert related search)
You can handle it in database trigger - again it will have concurrency problems
Avoiding concurrency problems completely requires hand written queries with locking hints to ensure that only one request can check number of searches per search list and insert a new search in atomic transaction.
I ended up going with CustomValidationAttribute, and implemented it with a great deal of success. See below for my implementation info:
In the SearchList entity
[NotMapped]
public String ValidationMessage { get; set; }
[CustomValidation(typeof(EntityValidation.EntityValidators), "ValidateSearchCount")]
public virtual List<Search> Searches { get; set; }
public static bool Create(ProjectContext db, SearchList searchList)
{
try
{
db.SearchLists.Add(searchList);
db.SaveChanges();
return true;
}
catch (DbEntityValidationException dbEx)
{
foreach (var validationErrors in dbEx.EntityValidationErrors)
{
foreach (var validationError in validationErrors.ValidationErrors)
{
searchList.ValidationMessage += validationError.ErrorMessage;
}
}
return false;
}
catch (Exception)
{
return false;
}
}
EntityValidators Class
public static ValidationResult ValidateSearchCount(List<Search> Searches)
{
bool isValid;
int count = Searches.Count();
isValid = (count <= 5000 ? true : false);
if (isValid)
{
return ValidationResult.Success;
}
else
{
return new ValidationResult("A maximum of 5000 searches may be added to a SearchList.");
}
}
A similar exception block is on the update method. In this way, when SaveChanges gets called it attempts to validate the entity and its child collections, and when the collection count is greater than 5000 the validator will return an error message which gets caught in the exception handler and stored in a local property for my controller to check when things go wrong.
I have a question for how to implement multiple criteria for a repository pattern in ASP.net MVC. Imagine a POCO class in EF4
public class people
{ String Name {get;set;}
float Height {get;set;}
float Weight {get;set;}
int age {get;set;}
....
}
If I build up a repository as IPeopleRepository, what kind of methods should I implement for a multiple criteria search (e.g Age > 30, Height >80). Those criteria would be related to the properties in the class and some of the input could be null. Of course I can write a method like
People SearchPeople (int age, float height.....)
but I have to judge if every variable would be null and append onto the search queries..
So do you have any good ideas on how to implement this function in EF?
It sounds like your looking for something like the Specification pattern.
There is a great article involving EF4 / POCO / Repository / Specification pattern here.
Although i like the pattern, i find it a bit overkill in simple scenarios.
I ended up using the "pipes and filters" technique - basically IQueryable<T> extension methods on your objects to make your repository code fluent.
For a search criteria however, i would be tempted to allow the consuming code to supply the predicate, then you don't have to worry about the parameters.
So the definition would be like this:
public People SearchPeople(Expression<Func<People,bool>> predicate)
{
return _context.People.SingleOrDefault(predicate);
}
Then the code simply supplies the predicate.
var person = _repository.SearchPeople(p => p.Age > 30 && p.Height > 80);
Some people don't like this technique, as it gives too much "power" to the consumer, because they might supply a predicate like p.Id > 0 and return all the rows in the database.
To counteract that, provide an optional parameter for maxRows. If it's not supplied, default to 100 rows.
First, you need to think if you really need repository search method.
You might want to do direct queries instead of wrapping them to repository.
However, if you think you need the search method than you will likely use something like this:
private People SearchPeople(int? age, float? height)
{
var baseQuery = db.People;
if (age != null)
baseQuery = baseQuery.Where(arg => arg.Age > age);
if (height != null)
baseQuery = baseQuery.Where(arg => arg.Height > height);
return baseQuery.ToList();
}
Although you didn't want to do this, I can't think of better solution.
Basically I think there are three options:
Use Specification pattern and create as many single specification as you need then you can more complex specification by combining them via And/Or/Not operators. You can look at here for an example http://code.google.com/p/linq-specifications/
Create a search method that accept an input predicate, it's most simple one since it leaves all criteria filtering works to consumers.
Create a search method with different criteria, then build dynamic Linq expression. There is a PredicateBuilder here: http://www.linqpad.net (look for LinqKit project).
I'm still trying to get my feet on the ground with Silverlight and RIA Services, and of course starting with some of the more "fun" stuff like grids and intelligent paging. I can connect to RIA Services (using a home-grown ORM, not L2S or EF), get data on the grid, and connect to a DataPager. The domain service is working well with the home-grown ORM, at least for queries. (Still working on full CRUD.) However, there are still problems:
To support the user application, I need user-controlled sorting and filtering, in addition to smart paging (only run the query for the rows needed to display) and grouping.
So far, I've seen nothing in the DataGrid or DataPager to externalize these capabilities so that filtering, sorting, and paging parameters can be passed to the server to build the appropriate query.
The datasets are potentially quite large; my table I've chosen for prototyping work can have up to 35,000 entries at some customers, and I'm sure there are other tables far larger that I will have to deal with at some point. So the "smart paging" aspect is essential.
Ideas, suggestions, guidance, and nerf bricks are all welcome.
OK, I've spent a few days in the weeds with this one, and I think I've got a handle on it.
First, an important piece of magic. For paging to work properly, the pager has to know the total item count, no matter how many items were returned by the current query. If the query returns everything, the item count is obviously the number of items returned. For smart paging, the item count is still the total of available items, although the query returns only what gets displayed. With filtering, even the total of available items changes every time the filter changes.
The Silverlight Datapager control has a property called ItemCount. It is readonly and cannot be databound in XAML, or set directly in code. However, if the user control containing the pager has a DataContext that implements IPagedCollectionView, then the data context object must implement an ItemCount property with PropertyChanged notification, and the DataPager seems to pick this up automagically.
Second, I highly recommend Brad Abrams' excellent series of blog posts on RIA Services, especially this one on ViewModel. It contains most of what you need to make paging and filtering work, although it's missing the critical piece on managing the item count. His downloadable sample also contains a very good basic framework for implementing ModelViewViewModel (MVVM). Thank you, Brad!
So here's how to make the item count work. (This code refers to a custom ORM, while Brad's code uses Entity Framework; between the two you can figure you what you need in your environment.)
First, your ORM needs to support getting record counts, with and without your filter. Here's my domain service code that makes the counts available to RIA Services:
[Invoke]
public int GetExamCount()
{
return Context.Exams.Count();
}
[Invoke]
public int GetFilteredExamCount(string descriptionFilter)
{
return Context.Exams.GetFilteredCount(descriptionFilter);
}
Note the [Invoke] attribute. You need this for any DomainService method that doesn't return an Entity or an Entity collection.
Now for the ViewModel code. You need an ItemCount, of course. (This is from Brad's example.)
int itemCount;
public int ItemCount
{
get { return itemCount; }
set
{
if (itemCount != value)
{
itemCount = value;
RaisePropertyChanged(ItemCountChangedEventArgs);
}
}
}
Your LoadData method will run the query to get the current set of rows for display in the DataGrid. (This doesn't implement custom sorting yet, but that's an easy addition.)
EntityQuery<ExamEntity> query =
DomainContext.GetPagedExamsQuery(PageSize * PageIndex, PageSize, DescriptionFilterText);
DomainContext.Load(query, OnExamsLoaded, null);
The callback method then runs the query to get the counts. If no filter is being used, we get the count for all rows; if there's a filter, then we get the count for filtered rows.
private void OnExamsLoaded(LoadOperation<ExamEntity> loadOperation)
{
if (loadOperation.Error != null)
{
//raise an event...
ErrorRaising(this, new ErrorEventArgs(loadOperation.Error));
}
else
{
Exams.MoveCurrentToFirst();
if (string.IsNullOrEmpty(DescriptionFilterText))
{
DomainContext.GetExamCount(OnCountCompleted, null);
}
else
{
DomainContext.GetFilteredExamCount(DescriptionFilterText, OnCountCompleted, null);
}
IsLoading = false;
}
}
There's also a callback method for counts:
void OnCountCompleted(InvokeOperation<int> op)
{
ItemCount = op.Value;
TotalItemCount = op.Value;
}
With the ItemCount set, the Datapager control picks it up, and we have paging with filtering and a smart query that returns only the records to be displayed!
LINQ makes the query easy with .Skip() and .Take(). Doing this with raw ADO.NET is harder. I learned how to do this by taking apart a LINQ-generated query.
SELECT * FROM
(select ROW_NUMBER() OVER (ORDER BY Description) as rownum, *
FROM Exams as T0 WHERE T0.Description LIKE #description ) as T1
WHERE T1.rownum between #first AND #last ORDER BY rownum
The clause "select ROW_NUMBER() OVER (ORDER BY Description) as rownum" is the interesting part, because not many people use "OVER" yet. This clause sorts the table on Description before assigning row numbers, and the filter is also applied before row numbers are assigned. This allows the outer SELECT to filter on row numbers, after sorting and filtering.
So there it is, smart paging with filtering, in RIA Services and Silverlight!
Here's the quick and dirty solution (that I went for):
Just move your DomainDataSource to your ViewModel! Done!
May not exactly be great for testability and probably some other limitations I haven't discovered yet, but personally I don't care about that until something better comes along.
Inside your ViewModel just instantiate the data source :
// Feedback DataSource
_dsFeedback = new DomainDataSource();
_dsFeedback.DomainContext = _razorSiteDomainContext;
_dsFeedback.QueryName = "GetOrderedFeedbacks";
_dsFeedback.PageSize = 10;
_dsFeedback.Load();
and provide a bindable property :
private DomainDataSource _dsFeedback { get; set; }
public DomainDataSource Feedback
{
get
{
return _dsFeedback;
}
}
And add your DataPager to your XAML:
<data:DataPager Grid.Row="1"
HorizontalAlignment="Stretch"
Source="{Binding Feedback.Data}"
Margin="0,0,0,5" />
<data:DataGrid ItemsSource="{Binding Feedback.Data}">
PS. Thanks to 'Francois' from the above linked page. I didn't even realize I could take the DomainDataSource out of the XAML until I saw your comment!
This is an interesting article from May 2010 about the possible future support for this type of feature in the framework.
http://www.riaservicesblog.net/Blog/post/WCF-RIA-Services-Speculation-EntityCollectionView.aspx