Lazy Load An Entity Framework EntityCollection with criteria

Question

In Entity Framework (specifically EF 3.5, but if it exists in EF 4 it gives me a reason to upgrade) is it possible to lazy load only part of a collection? I may be approaching this wrong too, so I'm open to suggestions. My tables/entities look similar to this:

Person            PersonMeal           Meal
------    1---*   ----------   *---1   -----
ID                ID                   ID
...               PersonID             ...
                  MealID
                  Value
                  ...

I have a list of Person objects that have been retrieved through Entity Framework via a stored procedure. I have a view that only shows one Meal at a time, so I only want the information related to that meal. Currently I have code that looks like this:

Function GetPersons() As List(Of Person)
    Dim personList = context.StoredProcedureCall(param1, param2, param3).ToList()
    personList.ForEach(Function(x) LazyLoadProperties(x))
    Return personList
End Function

' Work around function because VB lambdas don't take Sub's
Function LazyLoadProperties(ByVal person As Person) As Object
    If (Not person.PersonMeal.IsLoaded) Then
        person.PersonMeal.Load()
    End If
    Return Nothing
End Function

The issue is this is loading up the entire collection. Granted it's a small collection so worst case scenario I can load it all up and then remove all but the one I need, but that is far from ideal. Plus I'm not sure if it would be possible without triggering any of the events of modifying the collection since they shouldn't have been in there in the first place.

Does Person have a many-to-many relationship with PersonMeal and PersonMeal have a many-to-many relationship with Meal as shown in your question? Or does Person rather have a many-to-many relationship with Meal, while the PersonMeal is the connecting table? If so, there shouldn't be an entity generated for PersonMeal. I've also noticed a Value field, however. Could you clarify your data structure a bit? A screenshot of your EDM could be helpful, as well as your database structure. — Yakimych
– Yakimych, Commented Oct 15, 2010 at 18:18
@Yakimych You're right, it's a M2M between Person and Meal with PersonMeal being the connecting table/entity. — Agent_9191
– Agent_9191, Commented Oct 15, 2010 at 18:39
@Agent_9191 - Ok, in that case there should be just a PersonEntity table, but no entity generated. The Person entity should have a collection of Meals and the Meal entity should have a collection of People. Did you generate your model from the database or create it yourself in the designer? And what is the Value property for (that might be the reason EF is generating the PersonMeal entity)? — Yakimych
– Yakimych, Commented Oct 15, 2010 at 19:47
@Yakimych It was generated from the database, and the PersonMeal entity is necessary because there are properties specific to the combination (The type of food, beverage, vitamins, etc.). Value is one of the generic properties in the list. The issue is most views of data require just 1 meal at a time for a person and like I stated, the list of people is generated from a stored procedure so chaining LINQ Where clauses doesn't work. — Agent_9191
– Agent_9191, Commented Oct 15, 2010 at 20:04
@Agent_9191 - Ok, but your navigational property should be called PersonMeals rather than PersonMeal since it is a collection. Regarding your question: 'I have a view that only shows one Meal at a time, so I only want the information related to that meal'. How do you determine which meal exactly to load and show? — Yakimych
– Yakimych, Commented Oct 15, 2010 at 20:38

Yakimych · Accepted Answer · 2010-10-16 20:04:31Z

2

+300

In this case, instead of using the Load method, you can just query the database for your data:

Function GetPersons() As List(Of Person)
    Dim personList = context.StoredProcedureCall(param1, param2, param3).ToList()

    Dim person As Person
    For Each person in personList
        person.PersonMeals = From pm in context.PersonMeals.Include("Meal")
                             Where pm.Person.Id == person.Id And pm.Meal.Id == Meal_ID
                             Take 1
    Next person

    Return personList
End Function

I assume that person.PersonMeals is a collection, otherwise you can use FirstOrDefault instead of Take.

In this query we basically select all the PersonMeals entities (together with the Meal) that have the person ID as the current person in the loop and the meal ID you want. If your DB does not have corrupt data (multiple rows with the same PersonID-MealID combinations), there will be 0 or 1 results, which will be written into your PersonMeals property.

edited Oct 16, 2010 at 20:04

answered Oct 15, 2010 at 22:10

Yakimych

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1 Comment

Agent_9191 Over a year ago

Not as elegant of a solution as I was hoping for, but this works.

VdesmedT · Accepted Answer · 2010-10-21 12:31:11Z

1

Your question was pretty clear : is it possible to lazily load only part of a collection and the answer is no ! Not in EF1 nor in EF4. Btw, If the last meal is the subject, start querying by this ! Instead of retrieving the person and there meal, retrieve the last meals and the person attached to it.

answered Oct 21, 2010 at 12:31

VdesmedT

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2 Comments

Yakimych Over a year ago

Just to clarify - EF doesn't provide an out-of-the box method to lazy-load part of a collection. And lazy-loading means that the Meals collection doesn't get loaded together with the Person, but rather some time after the People are loaded. This you can clearly achieve by querying for the Meals, however. Thus, I wouldn't just plainly state that it's not possible.

Acaz Souza Over a year ago

Why the answer is No? I think is not hard to EF team to implement something to solve this problem.

p0wl · Accepted Answer · 2010-10-18 17:54:28Z

0

Yakimych's answer should work, but the code had a few mistakes.

Correct Syntax should be:

Private Function GetPersons() As List(Of Person)
    Dim personList As List(Of Person) = Context.StoredProcedureCall(param1, param2, param3).ToList()
    For Each p In personList
        Dim pId As Integer = p.Id
        p.PersonMeals = (From pm As PersonMeal In context.PersonMeals.Include("Meal")
                         Where (pm.Person.Id = pId And pm.Meal.Id = Meal_ID) Take 1).ToList
    Next
    Return personList
End Function

I hope it helps.

answered Oct 18, 2010 at 17:54

p0wl

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