Entity Framework 5, MVC 4, Code First - Persisting a List<string> in a class

Question

My solution should work well with: ASP.NET MVC 4, EF5, Code First, Visual Studio 2012 Express, SQL Server 2012 Express.

I need to store many tags with a place object. In another question it was suggested I use a List to achieve this:

asp.net MVC 4, tagging places - best practice (eg Pub, Store, Restaurant)

ie

public List<String> Tags {get;set;}

If I just add that it doesn't get persisted in the database at all. How can I make it persist?

Thanks.

Update

This needs a many to many relationship - eg a place might be tagged Cafe and Cycle Shop. There are many cafe's and cycle shops.

   public class Place
   {
      public Place()
      {
         // Set default value of dateAdded to now
         DateAdded = DateTime.Now;
      }

      [ScaffoldColumn(false)]
      public virtual int PlaceID { get; set; }

      public virtual List<Tag> Tags { get; set; }

      [ScaffoldColumn(false)]
      [DisplayName("Date Added")]
      public virtual DateTime DateAdded { get; set; }

      [Required(ErrorMessage = "Place Name is required")]
      [StringLength(100)]
      public virtual string Name { get; set; }
      public virtual string URL { get; set; }

      [DisplayName("Contact Name")]
      public virtual string ContactName { get; set; }
      public virtual string Address { get; set; }
      public virtual string City { get; set; }

      [DisplayName("Post Code")]
      public virtual string PostCode { get; set; }
      public virtual string Website { get; set; }
      public virtual string Phone { get; set; }

      [EmailAddress(ErrorMessage = "Please enter valid email")]
      public virtual string Email { get; set; }
      public virtual string About { get; set; }
      public virtual string Image { get; set; }
   }

Based on your description, you should show the code of your Place class at the very least as it sounds like it should have the collection of Tags. Is that your intent? — blins
– blins, Commented May 20, 2013 at 22:41

evanmcdonnal · Accepted Answer · 2013-05-20 22:38:01Z

5

How do you expect the Tags to be represented in the database? There is no native SQL Server type that EF maps to a List (it's more typical mapping is rows of a table to lists of objects) so you have to do some manipulation. If you want that to be a single cell (ie one field) then I would make it an nvarchar in the db and a string in code.

Then I would keep the list since it's easier to deal with in code and use the following LINQ query to transform the list into a comma separated list which will be a string, from there it can go into an nvarchar like any other string.

string csl = myList.Aggregate((c, n) => c + "," + n));

To go back to a list you just do;

List<string> = csl.Split(',').ToList();

Regardless of how you decide to represent your data it's not going to be as simple as just adding a list to a class. You'll have to put some thought into what makes the most sense for your application, this is just an idea to get you started since your question is a bit too vague to give a complete solution.

answered May 20, 2013 at 22:38

evanmcdonnal

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2 Comments

niico Over a year ago

Thanks - so it looks like it's between a comma separated string - or a separate tags table. Which would you recommend?

evanmcdonnal Over a year ago

@user2254951 that depends on the nature of tags. If there were an arbitrary limit or the expectation that a places tags list would never always be fairly short I would keep it on the places table as a csl. On the other hand, if it's boundless then it's necessary to have it on it's own table so the tags won't be limited by nvarchar(max).

Brent · Accepted Answer · 2013-05-20 23:00:00Z

3

As evanmcdonnal said, EF can't map a plain list of strings to a database. A simple solution would be to create a Tag entity:

public class Tag
{
    public Place Place { get; set; }
    public string Name { get; set; }
}

Then in your place entity, use a list of Tags:

public List<Tag> Tags { get; set; }

//just a helper, not required
public void AddTag(string tagName)
{
    Tag tag = new Tag { Name = tagName };
    Tags.Add(tag);
}

The result will be a new Tags table in your database that contains a foreign key back to the Places table.

answered May 20, 2013 at 23:00

Brent

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3 Comments

AaronLS Over a year ago

This solution will likely be easier to query for many scenarios than stuffing all the tags in a single field.

niico Over a year ago

As it's many to many I have added ICollection<Tag> Tags {get;set;} to Places, and ICollection<Place> Places {get;set;} to Tags.

blins Over a year ago

@niico, your use of ICollection is probably wise and possibly a better choice than List so that you are not coding to an implementation. See: stackoverflow.com/questions/7655845/…

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