Working with Entity Framework client vs server evaluation changes - entity-framework-core

Question

I have the following exceprt of my query. I am using ASP.NET Core 3.1 project with EF Core.
I read that the server vs client has changed, so how I used to perform the WHERE part in Core 2.1 (using variables from elsewhere in my code) doesn't seem to work anymore.
So as below I have changed (as per something I read) to use ToList() in each part, but now is it not hitting the database more (in my Core 2.1 I would of only had the ToList on the final part as per the code comment below).
So now for Core 3.1 I need to have a dynamic where in the initial "// Load data" part - how do I do a dynamic Where in the initial part, or is there a way, now the server vs client changes are in EF Core to work around that (note it is the final "// Search" part that fails in EF under Core 3.1 (prior to adding the ToList 's)
public List<KBEntryListVM> lstKBEntry;
// Load data
var q = await (from _k in _context.KBEntry
join _kc in _context.KBCategory on _k.CategoryId equals _kc.Id
into _kc2
from _kc3 in _kc2.DefaultIfEmpty()
select new KBEntryListVM()
{
Id = _k.Id,
DateCreated = DateTime.Parse(_k.DateCreated.ToString()),
CategoryId = _k.CategoryId,
CategoryTitle = _kc3.Title.ToString().Trim(),
Text = _k.Text.ToString().Trim(),
Title = _k.Title.ToString().Trim()
}).ToListAsync();
// KBCategory
if (!string.IsNullOrEmpty(c) && Guid.TryParse(c.ToString().Trim(), out var newGuid))
{
q = q.Where(w => w.CategoryId == Guid.Parse($"{c.ToString()}")).ToList();
}
// Search
if (!string.IsNullOrEmpty(s))
{
q = q.Where(w => w.Title.ToLower().Contains($"{s.ToLower()}") || w.CategoryTitle.ToLower().Contains($"{s.ToLower()}") || w.Text.ToLower().Contains($"{s.ToLower()}")).ToList();
}
lstKBEntry = q; //.ToList(); this would of been the only place in Core 2.1 I would of had ToList()
Arthur

Answer 1

So as below I have changed (as per something I read) to use ToList() in each part
EF Core 3.x+ client evaluation exception message suggests to either (1)
rewrite the query in a form that can be translated
or (2)
switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync()
So you are taking the option (2) which is easier, but you should try utilizing the option (1) which is harder, but better from performance perspective and the main reason implicit client evaluation have been removed by EFC 3.0. Option (2) should be your last resort only in case there is no way to apply option (1).
The exception message also contains the failed expression. Unfortunately it is not the exact part, but the whole expression (the whole Where predicate for instance), so you need to analyze it, find the failing part(s) and try to replace them with translatable constructs.
One of the general rules for simple data expressions is to avoid explicit conversions (ToString(), Parse). Store dates and numbers in database as such rather than strings, or utilize value conversions when using old existing database and aren't allowed to change it.
In this particular query, the unsupported (non translatable) construct most likely are ToString() calls of string type properties (e.g. Title, Text). EF Core still supports implicit client evaluation of the final Select, so you won't notice it if there is no Where (or other) clause after that referencing such expressions. But as being said at the beginning, you should avoid them regardless - query the raw data and let the usages (UI) do the desired formatting.
Anyway, I can't tell exactly because you haven't show your model, but removing ToString() should make the query translatable, hence no need of intermediate ToList() or similar client materialization:
CategoryTitle = _kc3.Title.Trim(),
Text = _k.Text.Trim(),
Title = _k.Title.Trim()
You should probably also replace
DateCreated = DateTime.Parse(_k.DateCreated.ToString())
with just
DateCreated = _k.DateCreated
because it seems that DateCreated is already DateTime, so the double conversion through string doesn't make sense and would cause similar troubles. And even if the database type is string, still remove Parse / ToString and setup value converter which does that.