return LINQ query to view and loop through the results in the view

Question

I am running in to a problem where I have made the following query in my controller:

var query = from pmt in db.ProjectHasTags
            join project in db.Projects on pmt.ProjectId equals project.ID
            join tag in db.ProjectTags
                 on pmt.TagId equals tag.ID
                 group pmt by pmt.Project into pmtGroup
                    select new
                    {
                        Project = pmtGroup.Key,
                        Tags = pmtGroup.Select(project => project.ProjectTag)
                    };

I want to return this query to a view using:

return View(query.ToList());

In the view file I have the following code:

@model IEnumerable<portfolio.Models.ProjectHasTag>

@foreach (var p in Model)
{
    @p.Project.Title

    foreach (var tag in p.Tags)
    {
        @tag.title
    }       
}

I get the following error:

The model item passed into the dictionary is of type 'System.Collections.Generic.List1[<>f__AnonymousType62[portfolio.Models.Project,System.Collections.Generic.IEnumerable1[portfolio.Models.ProjectTag]]]', but this dictionary requires a model item of type 'System.Collections.Generic.IEnumerable1[portfolio.Models.ProjectHasTag]'.

The ProjectHasTag model code:

public class ProjectHasTag
{
    public int ID { get; set; }

    public int? ProjectId { get; set; }

    [ForeignKey("ProjectId")]
    [DisplayName("Project")]
    public virtual Project Project { get; set; }

    public int? TagId { get; set; }

    [ForeignKey("TagId")]
    [DisplayName("Tag")]
    public virtual ProjectTag ProjectTag { get; set; }

    public virtual ICollection<ProjectTag> Tags { get; set; }

}

This is what I want to achieve: https://i.stack.imgur.com/DAZ5n.png (I cant post images yet)

thanks for taking the time, English is not my first language.

Answer 1

The problem is this part of your query:

select new
{
    Project = pmtGroup.Key,
    Tags = pmtGroup.Select(project => project.ProjectTag)
};

You're not specifying the type that should be instantiated, so it's creating an anonymous type, which is then being passed to your view. You probably want something like this:

select new ProjectHasTag
{
    Project = pmtGroup.Key,
    Tags = pmtGroup.Select(project => project.ProjectTag)
};

Update

Just as the error is telling you, ProjectHasTag does not have a Tags property. It looks like what you want is really this:

select new ProjectHasTag
{
    Project = pmtGroup.Key,
    ProjectTag = pmtGroup.Select(project => project.ProjectTag)
};

However, it's a little unclear what you're trying to do because in your view it looks as though you have multiple tags for each project, in which case it should really be a collection. Something like:

public virtual ICollection<ProjectTag> Tags { get; set; }

Update Two

I forgot the Entity Framework (EF) is picky when it comes to directly instantiating entity types. For a quick fix, you should be able to map from an anonymous type to the entity type (as described here).

However, EF doing this is actually a good thing because it's forcing you to adopt a design strategy that will allow you to leverage more power from MVC. In particular, this would be a good time to learn about ViewModels (see: ASP.NET MVC - How exactly to use View Models and http://geekswithblogs.net/michelotti/archive/2009/10/25/asp.net-mvc-view-model-patterns.aspx).

Answer 2

In the select part of your query, you create an anonymous object, and your model requires objects form type ProjectHasTag. So this should look like (look at the select new ProjectHasTag):

var query = from pmt in db.ProjectHasTags
            join project in db.Projects on pmt.ProjectId equals project.ID
            join tag in db.ProjectTags
                 on pmt.TagId equals tag.ID
                 group pmt by pmt.Project into pmtGroup
                    select new ProjectHasTag
                    {
                        Project = pmtGroup.Key,
                        Tags = pmtGroup.Select(project => project.ProjectTag)
                    };

Answer 3

Since you have an ICollection , you are telling Linq, that you want your ProjectTags to be lazy loaded. Like @John H said above, since you have added the Tags property to your ProjectHasTag class, you can populate it such:

select new ProjectHasTag
{
    Project = pmtGroup.Key,
    Tags = pmtGroup.Select(project => project.ProjectTag)
};