/// <summary>
/// Gets a page of size pageSize items, inside a db from dbFactory, and passes that to foreachBody(page, db).
///
/// Then closes the DbContext to keep EF from freaking out on long jobs, and does the above steps again.
///
/// Turns out to be the best way to handle processing a lot of items in EF. As an extra tweak, set a dirty flag at the top of the
/// loop body you pass in, flag it when you change anything, and call .SaveChanges() just the once as the loop body is ending if
/// the flag is set.
/// </summary>
/// <typeparam name="TDb"></typeparam>
/// <typeparam name="TModel"></typeparam>
/// <param name="pageSize"></param>
/// <param name="dbFactory"></param>
/// <param name="query"></param>
/// <param name="foreachBody">The body you'd normally place in a foreach loop for pages of items.</param>
public static void PagesOf <TDb, TModel>(int pageSize, BaseDbFactory <TDb> dbFactory, Func <TDb, IOrderedQueryable <TModel> > query, Action <IEnumerable <TModel>, TDb> foreachBody)
where TDb : DbContext, new()
{
int length = pageSize;
for (int page = 0; length == pageSize; page++)
{
using (var db = dbFactory.NewDb())
{
var items = query(db).Skip(page * pageSize).Take(pageSize).ToArray();
foreachBody(items, db);
length = items.Length;
}
}
}
/// <summary>
/// Pages through items, and detaches from the DbContext for each page; the body of your foreach loop will execute detached, meaning
/// no lazy loading or other EF access is possible - only the detached objects as-is are readable (no virtual navigation properties)
/// and no db is available for actions like .SaveChanges();
/// </summary>
/// <typeparam name="TDb"></typeparam>
/// <typeparam name="TModel"></typeparam>
/// <param name="pageSize"></param>
/// <param name="dbFactory"></param>
/// <param name="query"></param>
/// <returns></returns>
public static IEnumerable <TModel> EnumeratePagedDetached <TDb, TModel>(int pageSize, BaseDbFactory <TDb> dbFactory, Func <TDb, IOrderedQueryable <TModel> > query)
where TDb : DbContext, new()
{
int length = pageSize;
for (int page = 0; length == pageSize; page++)
{
TModel[] items;
using (var db = dbFactory.NewDb())
{
items = query(db).Skip(page * pageSize).Take(pageSize).ToArray();
}
foreach (var item in items)
{
yield return(item);
}
length = items.Length;
}
}
