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%'
Related
consider the following code:
function getPersonById(personId, type) {
var p1 = new breeze.Predicate("Id", "==", personId);
var p2 = new breeze.Predicate("Type", "==", type);
var query = breeze.EntityQuery.from("Contacts").where(p1.and(p2))
if (!manager.metadataStore.hasMetadataFor(service.serviceName)) {
return manager.fetchMetadata().then(function () {
return manager.executeQuery(query.using(service));
});
} else {
var fromCache = manager.getEntityByKey('Contact', personId);
if (fromCache)
return Q.resolve(fromCache);
return manager.executeQuery(query.using(service));
}
}
Am I doing things the right way ? It seems to me that I have to write a lot of boiler-plate code just for fetching an entity. I had to make sure the metadata was known, and then if the entity is already in cache or not.
I'm facing an issue because if executeQuery is called, then the return value is an array. However if getEntityByKey is called, then the return value is an Entity. How can I deal with that in an elegant way ? Is there a way to force executeQuery to return a single Entity rather than an array ? (I'm expecting only one returned value anyway)
Your metadata test shouldn't be necessary for each query. If you add a fail method that handles any errors (such as no metadata) you can write that only once, but in reality if whatever service type JavaScript file you are using is loaded metadata should always be there. If you are moving datacalls to the view models then I would recommend rethinking that strategy.
the way you are doing your cache check is optional. Remember that there are two ways to query from cache - executeQueryLocally and setting the fetchStrategy. There are some instances where you will want to go refresh data from the server so I would definitely recommend trying to pull from cache first in each query and only going to the database on a needed basis. Generally I only have two methods for all my data retrieval, for each entity, although if you are tricky you can probably reduce that to sharing queries as well. Your queries are most efficient when you can reuse them for different orderBy's, where clauses, etc...
Last, if you want to return only a single entity just do it lklike you would any other array - catch the returned array results before sending them back and filter it down to something like data.results[0]. You could also query and then use a filter to find the first entity that meets sine criteria.
I have two object, a room type and a reservation. Simplified they are:
class Room {
String description
int quantity
}
class Reservation {
String who
Room room
}
I want to query for all rooms along with the number of rooms available for each type. In SQL this does what I want:
select id, quantity, occupied, quantity-coalesce(occupied, 0) as available
from room left join(select room_id, count(room_id) as occupied from reservation)
on id = room_id;
I'm not getting anywhere trying to work out how to do this with HQL.
I'd appreciate any pointers since it seems like I'm missing something fairly fundamental in either HQL or GORM.
The problem here is your trying to represent fields that are not your domain classes like available and occupied. Trying to get HQL\GORM to do this can be a bit a little frustrating, but not impossible. I think you have a couple options here...
1.) Build your domain classes so that there easier to use. Maybe your Room needs to know about it's Reservations via a mapping table or, perhaps write what you want the code to look like and then adjust the design.
For example. Maybe you want your code to look like this...
RoomReservation.queryAllByRoomAndDateBetween(room, arrivalDate, departureDate);
Then you would implement it like this...
class RoomReservation{
...
def queryAllByRoomAndDateBetween(def room, Date arrivalDate, Date departureDate){
return RoomReservation.withCriteria {
eq('room', room)
and {
between('departureDate', arrivalDate, departureDate)
}
}
}
2.) My second thought is... It's okay to use the database for what it's good for. Sometimes using sql in you code is simply the most effective way to do something. Just do it in moderation and keep it centralized and unit tested. I don't suggest you use this approach because you query isn't that complex, but it is an option. I use stored procedures for things like 'dashboard view's' that query millions of objects for summary data.
class Room{
...
def queryReservations(){
def sql = new Sql(dataSoruce);
return sql.call("{call GetReservations(?)}", [this.id]) //<-- stored procedure.
}
}
I'm not sure how you can describe a left join with a subquery in HQL. INn any case you can easily execute raw SQL in grails too, if HQL is not expressive enough:
in your service, inject the dataSource and create a groovy.sql.Sql instance
def dataSource
[...]
def sql= new Sql(dataSource)
sql.eachRow("...."){row->
[...]
}
I know it's very annoying when people try to patronize you into their way of thinking when you ask a question, instead of answering your question or just shut up, but in my opinion, this query is sufficiently complex that I would create a concept for this number in my data structure, perhaps an Availability table associated to the Room, which would keep count not only of the quantity but also of the occupied value.
This is instead of computing it every time you need it.
Just my $.02 just ignore it if it annoys you.
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;
}
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.
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.