EntityFramework, Insert if not exist, otherwise up

问题:

I'm having a Entity-Set Countries, reflecting a database table '<'char(2),char(3),nvarchar(50> in my database.

Im having a parser that returns a Country[] array of parsed countries, and is having issues with getting it updated in the right way. What i want is: Take the array of countries, for those countries not already in the database insert them, and those existing update if any fields is different. How can this be done?

void Method(object sender, DocumentLoadedEvent e)
{
    var data = e.ParsedData as Country[];
    using(var db = new DataContractEntities)
    {
       //Code missing


    }
}

I was thinking something like

for(var c in data.Except(db.Countries)) but it wount work as it compares on wronge fields.

Hope anyone have had this issues before, and have a solution for me. If i cant use the Country object and insert/update an array of them easy, i dont see much benefict of using the framework, as from performers i think its faster to write a custom sql script that inserts them instead of ect checking if an country is already in the database before inserting?

Solution

See answer of post instead.

I added override equals to my country class:

    public partial class Country
{

    public override bool Equals(object obj)
    {
        if (obj is Country)
        {
            var country = obj as Country;
            return this.CountryTreeLetter.Equals(country.CountryTreeLetter);
        }
        return false;
    }
    public override int GetHashCode()
    {
        int hash = 13;
        hash = hash * 7 + (int)CountryTreeLetter[0];
        hash = hash * 7 + (int)CountryTreeLetter[1];
        hash = hash * 7 + (int)CountryTreeLetter[2];
        return hash;
    }
}

and then did:

        var data = e.ParsedData as Country[];
        using (var db = new entities())
        {
            foreach (var item in data.Except(db.Countries))
            {
                db.AddToCountries(item); 
            }
            db.SaveChanges();
        }

回答1:

I would do it straightforward:

void Method(object sender, DocumentLoadedEvent e)
{
    var data = e.ParsedData as Country[];
    using(var db = new DataContractEntities)
    {
        foreach(var country in data)
        {
            var countryInDb = db.Countries
                .Where(c => c.Name == country.Name) // or whatever your key is
                .SingleOrDefault();
            if (countryInDb != null)
                db.Countries.ApplyCurrentValues(country);
            else
                db.Countries.AddObject(country);
        }
        db.SaveChanges();
     }
}

I don't know how often your application must run this or how many countries your world has. But I have the feeling that this is nothing where you must think about sophisticated performance optimizations.

Edit

Alternative approach which would issue only one query:

void Method(object sender, DocumentLoadedEvent e)
{
    var data = e.ParsedData as Country[];
    using(var db = new DataContractEntities)
    {
        var names = data.Select(c => c.Name);
        var countriesInDb = db.Countries
            .Where(c => names.Contains(c.Name))
            .ToList(); // single DB query
        foreach(var country in data)
        {
            var countryInDb = countriesInDb
                .SingleOrDefault(c => c.Name == country.Name); // runs in memory
            if (countryInDb != null)
                db.Countries.ApplyCurrentValues(country);
            else
                db.Countries.AddObject(country);
        }
        db.SaveChanges();
     }
}

回答2:

The modern form, using later EF versions would be:

context.Entry(record).State = (AlreadyExists ? EntityState.Modified : EntityState.Added);
context.SaveChanges();

AlreadyExists can come from checking the key or by querying the database to see whether the item already exists there.

回答3:

You can implement your own IEqualityComparer<Country> and pass that to the Except() method. Assuming your Country object has Id and Name properties, one example of that implementation could look like this:

public class CountryComparer : IEqualityComparer<Country>
{
    public bool Equals(Country x, Country y)
    {
        return x.Name.Equals(y.Name) && (x.Id == y.Id);
    }

    public int GetHashCode(Country obj)
    {
        return string.Format("{0}{1}", obj.Id, obj.Name).GetHashCode();
    }
}

and use it as

data.Countries.Except<Country>(db, new CountryComparer());

Although, in your case it looks like you just need to extract new objects, you can use var newCountries = data.Where(c => c.Id == Guid.Empty); if your Id is Guid.

The best way is to inspect the Country.EntityState property and take actions from there regarding on value (Detached, Modified, Added, etc.)

You need to provide more information on what your data collection contains i.e. are the Country objects retrieved from a database through the entityframework, in which case their context can be tracked, or are you generating them using some other way.

回答4:

I am not sure this will be the best solution but I think you have to get all countries from DB then check it with your parsed data

 void Method(object sender, DocumentLoadedEvent e)
 {
    var data = e.ParsedData as Country[];
    using(var db = new DataContractEntities)
    {
       List<Country> mycountries = db.Countries.ToList();
       foreach(var PC in data)
       {
          if(mycountries.Any( C => C.Name==PC.Name ))
          {
             var country = mycountries.Any( C => C.Name==PC.Name );
             //Update it here
          }
          else
          {
               var newcountry = Country.CreateCountry(PC.Name);//you must provide all required parameters
               newcountry.Name = PC.Name;
               db.AddToCountries(newcountry)
          }
       }
       db.SaveChanges();
   }
  }