SaveChanges() outside of foreach saves only the last object added - sql

Question

Seems that the SaveChanges() only saves the last object added into the table:
static void Main(string[] args)
{
Category c = new Category();
using (GenericDBEntities db = new GenericDBEntities())
{
foreach (Match i in db.Matches)
{
if (!db.Categories.Any())
{
c.CategoryInternalId = i.CategoryId;
c.CategoryName = i.CategoryName;
c.SportId = i.SportId;
db.Categories.Add(c);
}
else
{
foreach (Category a in db.Categories)
{
if (i.CategoryId != a.CategoryInternalId)
{
c.CategoryInternalId = i.CategoryId;
c.CategoryName = i.CategoryName;
c.SportId = i.SportId;
db.Categories.Add(c);
}
else
{
return;
}
}
}
}
db.SaveChanges();
I have tried it in a few different ways which all have the same or less result, the for loop saves them all, no matter if condition is met or not. Why is it only saving the last object of the Match table? What am I missing?
Clarification: The task is to go through Match table, which has duplicate CategoryIDs and to, if it is not already in the Category table, add it, so it won't store duplicates again, the problem is Category table is initially empty, hence the newbie logic, still learnin'!

Answer 1

You need to create the Category object for each inserted category, otherwise you have just one category object which you are editing all the time and hence just this one (the last edit) will be saved to the DB.
If you want to add all new categories from Matches based on CategoryId you can use something like this:
using (GenericDBEntities db = new GenericDBEntities())
{
var newCategories = db.Matches
.Where(m => !db.Categories
.Select(c => c.CategoryInternalId)
.Distinct().Contains(m.CategoryId))
.Select(m => new { m.CategoryId, m.CategoryName, m.SportId })
.GroupBy(m => m.otherid)
.Select(g => g.FirstOrDefault())
.ToList();
foreach (var i in newCategories)
{
var c = new Category()
{
CategoryInternalId = i.CategoryId,
CategoryName = i.CategoryName,
SportId = i.SportId
};
db.Categories.Add(c);
}
db.SaveChanges();
}
Now you'll get all the new categories from the Matches table in one go. This is a first draft, meaning that the performance can be further tuned if needed.