Entity Framework & MVC - Insert into multiple table using foreign keys

Question

I have two tables, Property and PropertyMeta. They are linked by the PropertyID value. I have just started using ASP.net MVC and am currently using MVC2 (due to hosting provider contraints). From a simple create page i am trying to populate both tables but i'm not having much luck at all.

[HttpPost, Authorize]
    public ActionResult Create(PropertyViewModel property)
    {
        try
        {

            MyEntities db = new MyEntities();

            Property _property = new Property();
            _property = property.Properties;

            PropertyMeta _meta = new PropertyMeta();
            _meta.Property = _property;
            _meta = property.Meta;

            db.AddToProperties(_property);
            db.AddToPropertyMetas(_meta);
            db.SaveChanges();   

            return RedirectToAction("Index");
        }
        catch
        {
            return View();
        }
    }

I have tried the above but it isn't working. It's annoying, this should be so simple but i'm stuck. Can anyone think what might be wrong with the above? Any help would be greatly appreciated!

It isn't saving any data at all and giving "No parameterless constructor defined for this object." — Michael Willmott
– Michael Willmott, Commented Sep 13, 2011 at 9:48
FYI, standard practice is to only use _'s for private member variables, not for local variables. Also, when you say you can't use MVC 3, why is that? You don't have to have your hosting provider install MVC, you can do a bin deployment. The only reason you couldn't is if they don't have .net 4 installed. — Erik Funkenbusch
– Erik Funkenbusch, Commented Sep 15, 2011 at 6:30

Jeff Ogata · Accepted Answer · 2011-09-12 22:23:58Z

1

Is PropertyViewModel.Properties of type Property? I'm guessing the problem is in these 2 lines of code, since it doesn't seem to make sense to create a new Property and immediately assign property.Properties to it.

Property _property = new Property();
_property = property.Properties;

answered Sep 12, 2011 at 22:23

Jeff Ogata

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1 Comment

Michael Willmott Over a year ago

Yes, i should have made that clearer. I have created a viewmodel that consists of the property and propertymeta types. So if i assign property.Properties does that not take the properties fields from the create page and populate the _property variable with them? Thanks for your help.

benjamin · Accepted Answer · 2011-09-17 00:26:20Z

0

you are overwriting your value.

_meta.Property = _property; //sets an individual property
_meta = property.Meta; //Overwrites all properties

reverse the lines so you write all the values and then set the individual one after.

_meta = property.Meta; //Overwrites all properties
_meta.Property = _property; //sets an individual property

After reading your comment I realized what you were trying to do. Your property has no ID until after it has been entered into the DB. You can see this if you step through it in debugger. To fix this first submit the property before applying it to your meta.

        MyEntities db = new MyEntities();

        Property _property = property.Properties;
        db.AddToProperties(_property);
        db.SaveChanges();

At this point if you look in the debugger _properties ID value will now be set to the value assigned by the database. Now when you tie your meta to the property it will be an actual property item with a key so it will know how to associate it in the DB.

        PropertyMeta _meta = property.Meta;
        _meta.Property = _property;            
        db.AddToPropertyMetas(_meta);
        db.SaveChanges();

edited Sep 17, 2011 at 0:26

answered Sep 12, 2011 at 22:40

benjamin

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2 Comments

Michael Willmott Over a year ago

Thanks for the suggestion, it still isn't working though. In the entities model it has values of _property.PropertyMeta and _meta.Property for the foreign key (apologies if this isn't making sense). I was under the impression that i need to set one equal to the other during the create statement so both records are created with the same PropertyID.

benjamin Over a year ago

I'm sorry I didn't quite understand what you were attempting. You are creating a new property and a new meta but there is no ID created yet because the ID I take it is auto-generated by the database. I will modify my answer. You need to create the property first then create the meta so the foreign key has an actual value.

Erik Funkenbusch · Accepted Answer · 2011-09-15 06:25:03Z

Your problem is not that Entity Framework is too complex, you seem to have a poor grasp of how C# works in general, given that you're creating new variables and immediately overwriting them with values of different types (not even sure how that compiles).

You have several problems:

Property _property = new Property(); 
_property = property.Properties;

This creates a new _property. Then you assign a new Property over the one you just created. You shouldn't be using a domain model object in your viewmodel, but that's not really your problem.

You can just do this:

Property _property = property.Properties;

Next, you have this problem:

PropertyMeta _meta = new PropertyMeta();      
_meta.Property = _property; 
_meta = property.Meta; // overwrites the value assigned above

Again, you're overwriting the objects you just created. So this is equivelent:

PropertyMeta _meta = property.Meta;
_meta.Property = _property;

Finally, here's the last bit:

db.PropertyMetas.Add(_meta);             
db.SaveChanges();

EF will automatically add the Property to the table because it's linked in your _meta.Property table, so you don't add it explicitly.

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