Code re-use with Linq-to-Sql - Creating 'generic' look-up tables - asp.net-mvc

I'm working on an application at the moment in ASP.NET MVC which has a number of look-up tables, all of the form
LookUp {
Id
Text
}
As you can see, this just maps the Id to a textual value. These are used for things such as Colours. I now have a number of these, currently 6 and probably soon to be more.
I'm trying to put together an API that can be used via AJAX to allow the user to add/list/remove values from these lookup tables, so for example I could have something like:
http://example.com/Attributes/Colours/[List/Add/Delete]
My current problem is that clearly, regardless of which lookup table I'm using, everything else happens exactly the same. So really there should be no repetition of code whatsoever.
I currently have a custom route which points to an 'AttributeController', which figures out the attribute/look-up table in question based upon the URL (ie http://example.com/Attributes/Colours/List would want the 'Colours' table). I pass the attribute (Colours - a string) and the operation (List/Add/Delete), as well as any other parameters required (say "Red" if I want to add red to the list) back to my repository where the actual work is performed.
Things start getting messy here, as at the moment I've resorted to doing a switch/case on the attribute string, which can then grab the Linq-to-Sql entity corresponding to the particular lookup table. I find this pretty dirty though as I find myself having to write the same operations on each of the look-up entities, ugh!
What I'd really like to do is have some sort of mapping, which I could simply pass in the attribute name and get out some form of generic lookup object, which I could perform the desired operations on without having to care about type.
Is there some way to do this to my Linq-To-Sql entities? I've tried making them implement a basic interface (IAttribute), which simply specifies the Id/Text properties, however doing things like this fails:
System.Data.Linq.Table<IAttribute> table = GetAttribute("Colours");
As I cannot convert System.Data.Linq.Table<Colour> to System.Data.Linq.Table<IAttribute>.
Is there a way to make these look-up tables 'generic'?
Apologies that this is a bit of a brain-dump. There's surely imformation missing here, so just let me know if you'd like any further details. Cheers!

You have 2 options.
Use Expression Trees to dynamically create your lambda expression
Use Dynamic LINQ as detailed on Scott Gu's blog
I've looked at both options and have successfully implemented Expression Trees as my preferred approach.
Here's an example function that i created: (NOT TESTED)
private static bool ValueExists<T>(String Value) where T : class
{
ParameterExpression pe = Expression.Parameter(typeof(T), "p");
Expression value = Expression.Equal(Expression.Property(pe, "ColumnName"), Expression.Constant(Value));
Expression<Func<T, bool>> predicate = Expression.Lambda<Func<T, bool>>(value, pe);
return MyDataContext.GetTable<T>().Where(predicate).Count() > 0;
}

Instead of using a switch statement, you can use a lookup dictionary. This is psuedocode-ish, but this is one way to get your table in question. You'll have to manually maintain the dictionary, but it should be much easier than a switch.
It looks like the DataContext.GetTable() method could be the answer to your problem. You can get a table if you know the type of the linq entity that you want to operate upon.
Dictionary<string, Type> lookupDict = new Dictionary<string, Type>
{
"Colour", typeof(MatchingLinqEntity)
...
}
Type entityType = lookupDict[AttributeFromRouteValue];
YourDataContext db = new YourDataContext();
var entityTable = db.GetTable(entityType);
var entity = entityTable.Single(x => x.Id == IdFromRouteValue);
// or whatever operations you need
db.SubmitChanges()
The Suteki Shop project has some very slick work in it. You could look into their implementation of IRepository<T> and IRepositoryResolver for a generic repository pattern. This really works well with an IoC container, but you could create them manually with reflection if the performance is acceptable. I'd use this route if you have or can add an IoC container to the project. You need to make sure your IoC container supports open generics if you go this route, but I'm pretty sure all the major players do.

Related

use ToListAsync() with navigation property

I am using ASP.NET MVC with Entity Framework.
I have an entity called "AllUserData".
I have a second entity called "Genres". Each row in the table is a genre.
These two entities have a one-to-many relationship. So in the class definition of the "AllUserData" class, I have
public virtual ICollection PreferredGenres { get; set; }
I am able to successfully read the genres preferred by each user using
AllUserData aud = db.AllUserData.Single(b => b.UserId == currentUserId);
var chosengenres = aud.PreferredGenres.ToList()
However I cannot use
AllUserData aud = db.AllUserData.Single(b => b.UserId == currentUserId);
var chosengenres = await aud.PreferredGenres.ToListAsync()
Visual Studio says "ICollection does not contain a definition for 'ToListAsync' and the best extension method overload 'QueryableExtensions.ToListAsync(IQueryable)' requires a receiver of type 'IQueryable'.
Why is this happening? The only difference between the two was that in one case I used ToList() and in the other I used ToListAsync().
Can async methods not be used with navigation properties? In a real life application there are many cases where there are relationships between various entities; can asynchronous methods not be used when accessing properties using these relationships? Is there some way around this? I'd rather do things asynchronously if possible.
Your navigation property would have been declared as an ICollection, standard for Entity properties -- the compiler won't be pleased.
However, you can get an IQueryable with AsQueryable or with another Select() (if that makes sense for your needs) between the navigation collection and the ToListAsync().
ToList() can operate on IEnumerable (which includes ICollection and IQueryable). ToListAsync() has only been made to work with IQueryable, hence the message from VS. The "why" may lie in the implementation details.
Ok, it seems entity framework doesn't allow for calling the navigation property asynchronously in this manner.
What worked for me is earger loading the navigation property in the beginning. (Thanks to user daf for sending me in the correct direction).
So instead of
AllUserData aud = await db.AllUserData.SingleAsync(b => b.UserId == currentUserId);
var usergenres = await aud.PreferredGenres.ToListAsync();
which does not work, I can instead do this :
AllUserData aud = await db.AllUserData.Include(p=>p.PreferredGenres).SingleAsync(b => b.UserId == currentUserId);
var usergenres = aud.PreferredGenres.ToList();
This way the first query asks to pull all necessary information from the database through eager loading with .Include(), and does so asynchronously due to the user of .SingleAsync(). The next statement uses .ToList(), but shouldn't make a second trip to the database since the data is already eager loaded, so it doesn't matter that it is async. The whole operation is now async.
I also realized this same question has been asked previously on StackOverflow; it is a little difficult to find depending on what search terms you used: Entity Framework Designer First get navigation property as Tasks. The selected answer provides 4 different solutions, out of which the first one is similar to what I am using now.
It also appears possible to designate navigation properties as async when defining the entity class- see the section titled "Async Lazy Loading" on this page: here The sample snippet demonstrates how to do this for a single navigation property; I don't know if it can be done for an ICollection navigation property as well, which is what I need, but I didn't do any further digging either.

grails: how to properly edit/update a collection?

I just wasted half a day trying to figure this out, reading about some workarounds, and thinking "it can't be that bad - there must be a straightforward to do edit a collection in Grails, whethere using scaffolded views or my own."
Let's say I have this domain object:
class TreeGroup {
String name
List<Tree> trees
static hasMany = ['trees': MyTree]
}
Just to explain the choice of data structure - I need my records to be unique, but in the order I set. That's why I chose List, AFAIK one cannot rely on order in a Set. So there are 2 pieces to this question - 1) how to remove from any Collection, for example a Set, 2) is List the best replacement for Set in this context (preserving order).
I want to be able to create a group record with no trees in it and make 4 updates:
edit/save
edit the group record to reference 2 trees A and B
add another tree C
remove A
remove B and C
And obviously, I want the desired state after every step. Currently though, I can only add records, and if I even edit/save to list, the list elements are added to it again.
I am using the multiple select tag for this. It looks like this:
<g:select name="trees" from="${allTrees}" optionKey="id"
multiple="true" class="many-to-many"
value="${trees ? trees*.id : treeGroupInstance?.trees*.id}" />
and that's fine, in the sense that it generates an HTTP header with these variables on update:
_method:PUT
version:19
name:d5
trees:1
_action_update:Update
But the data binder only adds new elements, it never lets you edit a list.
What is the cleanest way to do it ? Is it me, not reading something obvious, or is this a design flaw of grails data binding (and of so, when/how will it be fixed) ?
Is there a way perhaps via a hidden HTTP parameter to clear the list before (re)adding elements ?
Thanks
I ended up doing this:
private repopulate(def domainObject, String propertyName, Class domainKlaz) {
if (params[propertyName] != null) {
domainObject[propertyName].clear()
domainObject[propertyName].addAll(
params[propertyName].collect { domainKlaz.get(it) }
)
}
}
and I am calling it in update controller method before save(), for every collection. OMG how ugly.

How to move from untyped DataSets to POCO\LINQ2SQL in legacy application

Good day!
I've a legacy application where data access layer consists of classes where queries are done using SqlConnection/SqlCommand and results are passed to upper layers wrapped in untyped DataSets/DataTable.
Now I'm working on integrating this application into newer one where written in ASP.NET MVC 2 where LINQ2SQL is used for data access. I don't want to rewrite fancy logic of generating complex queries that are passed to SqlConnection/SqlCommand in LINQ2SQL (and don't have permission to do this), but I'd like to have result of these queries as strong-typed objects collection instead of untyped DataSets/DataTable.
The basic idea is to wrap old data access code in a nice-looking from ASP.NET MVC "Model".
What is the fast\easy way of doing this?
Additionally to the answer below here is a nice solution based on AutoMapper: http://elegantcode.com/2009/10/16/mapping-from-idatareaderidatarecord-with-automapper/
An approach that you could take is using the DataReader and transfer. So for every object you want to work with define the class in a data transfer object folder (or however your project is structured) then in you data access layer have something along the lines of the below.
We used something very similar to this in a project with a highly normalized database but in the code we did not need that normalization so we used procedures to put the data into more usable objects. If you need to be able to save these objects as well you will need handle translating the objects into database commands.
What is the fast\easy way of doing
this?
Depending on the number of classes etc this is could not be the fastest approach but it will allow you to use the objects very similarly to the Linq objects and depending on the type of collections used (IList, IEnumerable etc) you will be able to use the extension methods on those types of collections.
public IList<NewClass> LoadNewClasses(string abc)
{
List<NewClass> newClasses = new List<NewClass>();
using (DbCommand command = /* Get the command */)
{
// Add parameters
command.Parameters["#Abc"].Value = abc;
// Could also put the DataReader in a using block
IDataReader reader = /* Get Data Reader*/;
while (reader.Read())
{
NewClass newClass = new NewClass();
newClass.Id = (byte)reader["Id"];
newClass.Name = (string)reader["Name"];
newClasses.Add(newClass);
}
reader.Close();
}
return newClasses;
}

How can I use nHibernate to get a small subset of object properties for list view

I am using ASP.NET-MVC and nHibernate for the first time. Great tools, but big learning curve!
I have a list view of objects that are quite large (each has about 60 properties). In the list view I am only using about 10 of these properties. Performance is not too bad, but it seems a waste to fully hydrate these objects. What is the recommended practice?
I tried using HQL to select fewer properties, but it seems like it won't partially hydrate an object. I also tried making property on the main object that was a header class with the list view properties, but I couldn't seem to get it mapped properly. I think this should be easy but I've really been struggling with it.
EDIT:
I kept coming back to this because I knew Anton had given me the answer and I just couldn't see it.
There are three things you need to do:
Make an object with the properties you want.
Make a mapping file to import this object.
<hibernate-mapping
xmlns="urn:nhibernate-mapping-2.2"
namespace="Core.Entities"
assembly="Core"
default-access="property">
<import class="RequestHeader" />
</hibernate-mapping>
If you are using HQL, your object must contain a constructor with all the properties, in the same order, as your select new statement. If you use the Criteria API, you don't need to do this.
public IList<RequestHeader> ListAll()
{
using (ISession session = GetSession())
{
using (ITransaction tx = session.BeginTransaction())
{
IList<RequestHeader> results = session.CreateCriteria(typeof (Request), "r")
.CreateCriteria("Requestor", "req", JoinType.InnerJoin)
.CreateCriteria("r.Grant", "g", JoinType.InnerJoin)
.SetProjection(Projections.ProjectionList()
.Add(Projections.Property("r.Id"), "Id")
.Add(Projections.Property("r.Status"), "Status")
.Add(Projections.Property("r.SubmissionDate"), "SubmissionDate")
.Add(Projections.Property("req.Name"), "Requestor")
.Add(Projections.Property("g.Number"), "Number"))
.SetResultTransformer(Transformers.AliasToBean(typeof (RequestHeader)))
.SetMaxResults(10000)
.List<RequestHeader>();
tx.Commit();
return results;
}
}
}
60 properties is too much. See Component mapping.
As for selecting a subset of properties, see this: you need a select new HQL construct. Be aware, though, that you need an appropriate constructor and that an object you'll get cannot be saved back to the DB.

Repository Interface - Available Functions & Filtering Output

I've got a repository using LINQ for modelling the data that has a whole bunch of functions for getting data out. A very common way of getting data out is for things such as drop down lists. These drop down lists can vary. If we're creating something we usually have a drop down list with all entries of a certain type, which means I need a function available which filters by the type of entity. We also have pages to filter data, the drop down lists only contain entries that currently are used, so I need a filter that requires used entries. This means there are six different queries to get the same type of data out.
The problem with defining a function for each of these is that there'd be six functions at least for every type of output, all in one repository. It gets very large, very quick. Here's something like I was planning to do:
public IEnumerable<Supplier> ListSuppliers(bool areInUse, bool includeAllOption, int contractTypeID)
{
if (areInUse && includeAllOption)
{
}
else if (areInUse)
{
}
else if (includeAllOption)
{
}
}
Although "areInUse" doesn't seem very English friendly, I'm not brilliant with naming. As you can see, logic resides in my data access layer (repository) which isn't friendly. I could define separate functions but as I say, it grows quite quick.
Could anyone recommend a good solution?
NOTE: I use LINQ for entities only, I don't use it to query. Please don't ask, it's a constraint on the system not specified by me. If I had the choice, I'd use LINQ, but I don't unfortunately.
Have your method take a Func<Supplier,bool> which can be used in Where clause so that you can pass it in any type of filter than you would like to construct. You can use a PredicateBuilder to construct arbitrarily complex functions based on boolean operations.
public IEnumerable<Supplier> ListSuppliers( Func<Supplier,bool> filter )
{
return this.DataContext.Suppliers.Where( filter );
}
var filter = PredicateBuilder.False<Supplier>();
filter = filter.Or( s => s.IsInUse ).Or( s => s.ContractTypeID == 3 );
var suppliers = repository.ListSuppliers( filter );
You can implement
IEnumerable<Supplier> GetAllSuppliers() { ... }
end then use LINQ on the returned collection. This will retrieve all suppliers from the database that are then filtered using LINQ.
Assuming you are using LINQ to SQL you can also implement
IQueryable<Supplier> GetAllSuppliers() { ... }
end then use LINQ on the returned collection. This will only retrieve the necessary suppliers from the database when the collection is enumerated. This is very powerful and there are also some limits to the LINQ you can use. However, the biggest problem is that you are able to drill right through your data-access layer and into the database using LINQ.
A query like
var query = from supplier in repository.GetAllSuppliers()
where suppliers.Name.StartsWith("Foo") select supplier;
will map into SQL similar to this when it is enumerated
SELECT ... WHERE Name LIKE 'Foo%'

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