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How to avoid adding a reference field in very small entries (thus avoiding doubling collection size)?

I have a User class that accumulates lots of DataTime entries in some List<DateTime> Entries field.
Occasionally, I need to get last 12 Entries (or less, if not reached to 12). It can get to very large numbers.
I can add new Entry object to dedicated collection, but then I have to add ObjectId User field to refer the related user.
It seems like a big overhead, for each entry that holds only a DateTime, to add another field of ObjectId. It may double the collection size.
As I occasionally need to quickly get only last 12 entries of 100,000 for instance, I cannot place these entries in a per-user collection like:
class PerUserEntries {
public ObjectId TheUser;
public List<DateTime> Entries;
}
Because it's not possible to fetch only N entries from an embedded array in a mongo query, AFAIK (if I'm wrong, it would be very gladdening!).
So am I doomed to double my collection size or is there a way around it?
Update, according to #profesor79's answer:
If your answer works, that will be perfect! but unfortunately it fails...
Since I needed to filter on the user entity as well, here is what I did:
With this data:
class EndUserRecordEx {
public ObjectId Id { get; set; }
public string UserName;
public List<EncounterData> Encounters
}
I am trying this:
var query = EuBatch.Find(u => u.UserName == endUser.UserName)
.Project<BsonDocument>(
Builders<EndUserRecordEx>.Projection.Slice(
u => u.Encounters, 0, 12));
var queryString = query.ToString();
var requests = await query.ToListAsync(); // MongoCommandException
This is the query I get in queryString:
find({ "UserName" : "qXyF2uxkcESCTk0zD93Sc+U5fdvUMPow" }, { "Encounters" : { "$slice" : [0, 15] } })
Here is the error (the MongoCommandException.Result):
{
{
"_t" : "OKMongoResponse",
"ok" : 0,
"code" : 9,
"errmsg" : "Syntax error, incorrect syntax near '17'.",
"$err" : "Syntax error, incorrect syntax near '17'."
}
}
Update: problem identified...
Recently, Microsoft announced their DocumentDB protocol support for MongoDB. Apparently, it doesn't support yet all projection operators. I tried it with mLab.com, and it works.

You can use PerUserEntries as this is a valuable document structure.
To get part of that array we need to add projection to query, so we can get only x elements and this is done server side.
Please see snippet below:
static void Main(string[] args)
{
// To directly connect to a single MongoDB server
// or use a connection string
var client = new MongoClient("mongodb://localhost:27017");
var database = client.GetDatabase("test");
var collection = database.GetCollection<PerUserEntries>("tar");
var newData = new PerUserEntries();
newData.Entries = new List<DateTime>();
for (var i = 0; i < 1000; i++)
{
newData.Entries.Add(DateTime.Now.AddSeconds(i));
}
collection.InsertOne(newData);
var list =
collection.Find(new BsonDocument())
.Project<BsonDocument>
(Builders<PerUserEntries>.Projection.Slice(x => x.Entries, 0, 3))
.ToList();
Console.ReadLine();
}
public class PerUserEntries
{
public List<DateTime> Entries;
public ObjectId TheUser;
public ObjectId Id { get; set; }
}

Getting Missing Rows inside a table and copy them to another table

var FileProducts = from ProductsRow in ProductRangesDt.AsEnumerable()
join Filee in FileTb.AsEnumerable() on ProductsRow["GEN_CODE"].ToString() equals Filee["GEN_CODE"].ToString()
select new
{
PRODUCT_ID = ProductsRow["PRODUCT_ID"],
PRODUCT_NAME = ProductsRow["PRODUCT_NAME"],
PROVIDER_ID = ProductsRow["PROVIDER_ID"],
PROVIDER_NAME = ProductsRow["PROVIDER_NAME"],
GEN_CODE = ProductsRow["GEN_CODE"],
MIN_QUANTITY = Filee["MIN_QUANTITY"],
MAX_QUANTITY = Filee["MAX_QUANTITY"],
DISCOUNT_VALUE = Filee["DISCOUNT_VALUE"]
};
var s = (from b in FileProducts
select b.PRODUCT_ID).Distinct(); // count=285
var Products = (from ProductsRow in ProductRangesDt.AsEnumerable()
select ProductsRow["PRODUCT_ID"]).Distinct(); // count=7159
var result = Products.Except(s); // it's count should be 7159-285
I want to get all the products ID that are in Products and don't exist in FileProducts how can i do this ? result always return 0 as count

From the MSDN documentation about Except extension method:
This method is implemented by using deferred execution. The immediate
return value is an object that stores all the information that is
required to perform the action. The query represented by this method
is not executed until the object is enumerated either by calling its
GetEnumerator method directly or by using foreach in Visual C# or For
Each in Visual Basic.
So in order to get the real value form your Set differentiation, you need to enumerate your result either by a call to the Count()-Method (result.Count()) on using foreach (foreach (var r in result) { ... }).
I can't test with your data, but with test data at my disposition, the Except-extension did delivered the correct results.

Updating entire node with mutating cypher in Neo4jclient

I need to update all the properties of a given node, using mutating cypher. I want to move away from Node and NodeReference because I understand they are deprecated, so can't use IGraphClient.Update. I'm very new to mutating cypher. I'm writing in C#, using Neo4jclient as the interface to Neo4j.
I did the following code which updates the "Name" property of a "resunit" where property "UniqueId" equals 2. This works fine. However,
* my resunit object has many properties
* I don't know which properties have changed
* I'm trying to write code that will work with different types of objects (with different properties)
It was possible with IGraphClient.Update to pass in an entire object and it would take care of creating cypher that sets all properies.
Can I somehow pass in my object with mutating cypher as well?
The only alternative I can see is to reflect over the object to find all properties and generate .Set for each, which I'd like to avoid. Please tell me if I'm on the wrong track here.
string newName = "A welcoming home";
var query2 = agencyDataAccessor
.GetAgencyByKey(requestingUser.AgencyKey)
.Match("(agency)-[:HAS_RESUNIT_NODE]->(categoryResUnitNode)-[:THE_UNIT_NODE]->(resunit)")
.Where("resunit.UniqueId = {uniqueId}")
.WithParams(new { uniqueId = 2 })
.With("resunit")
.Set("resunit.Name = {residentialUnitName}")
.WithParams(new { residentialUnitName = newName });
query2.ExecuteWithoutResults();

It is indeed possible to pass an entire object! Below I have an object called Thing defined as such:
public class Thing
{
public int Id { get; set; }
public string Value { get; set; }
public DateTimeOffset Date { get; set; }
public int AnInt { get; set; }
}
Then the following code creates a new Thing and inserts it into the DB, then get's it back and updates it just by using one Set command:
Thing thing = new Thing{AnInt = 12, Date = new DateTimeOffset(DateTime.Now), Value = "Foo", Id = 1};
gc.Cypher
.Create("(n:Test {thingParam})")
.WithParam("thingParam", thing)
.ExecuteWithoutResults();
var thingRes = gc.Cypher.Match("(n:Test)").Where((Thing n) => n.Id == 1).Return(n => n.As<Thing>()).Results.Single();
Console.WriteLine("Found: {0},{1},{2},{3}", thingRes.Id, thingRes.Value, thingRes.AnInt, thingRes.Date);
thingRes.AnInt += 100;
thingRes.Value = "Bar";
thingRes.Date = thingRes.Date.AddMonths(1);
gc.Cypher
.Match("(n:Test)")
.Where((Thing n) => n.Id == 1)
.Set("n = {thingParam}")
.WithParam("thingParam", thingRes)
.ExecuteWithoutResults();
var thingRes2 = gc.Cypher.Match("(n:Test)").Where((Thing n) => n.Id == 1).Return(n => n.As<Thing>()).Results.Single();
Console.WriteLine("Found: {0},{1},{2},{3}", thingRes2.Id, thingRes2.Value, thingRes2.AnInt, thingRes2.Date);
Which gives:
Found: 1,Foo,12,2014-03-27 15:37:49 +00:00
Found: 1,Bar,112,2014-04-27 15:37:49 +00:00
All properties nicely updated!

How do I order a sql datasource of uniqueidentifiers in Linq by an array of uniqueindentifiers

I have a string list(A) of individualProfileId's (GUID) that can be in any order(used for displaying personal profiles in a specific order based on user input) which is stored as a string due to it being part of the cms functionality.
I also have an asp c# Repeater that uses a LinqDataSource to query against the individual table. This repeater needs to use the ordered list(A) to display the results in the order specified.
Which is what i am having problems with. Does anyone have any ideas?
list(A)
'CD44D9F9-DE88-4BBD-B7A2-41F7A9904DAC',
'7FF2D867-DE88-4549-B5C1-D3C321F8DB9B',
'3FC3DE3F-7ADE-44F1-B17D-23E037130907'
Datasource example
IndividualProfileId Name JobTitle EmailAddress IsEmployee
3FC3DE3F-7ADE-44F1-B17D-23E037130907 Joe Blo Director dsd#ad.com 1
CD44D9F9-DE88-4BBD-B7A2-41F7A9904DAC Maxy Dosh The Boss 1
98AB3AFD-4D4E-4BAF-91CE-A778EB29D959 some one a job 322#wewd.ocm 1
7FF2D867-DE88-4549-B5C1-D3C321F8DB9B Max Walsh CEO 1

There is a very simple (single-line) way of doing this, given that you get the employee results from the database first (so resultSetFromDatabase is just example data, you should have some LINQ query here that gets your results).
var a = new[] { "GUID1", "GUID2", "GUID3"};
var resultSetFromDatabase = new[]
{
new { IndividualProfileId = "GUID3", Name = "Joe Blo" },
new { IndividualProfileId = "GUID1", Name = "Maxy Dosh" },
new { IndividualProfileId = "GUID4", Name = "some one" },
new { IndividualProfileId = "GUID2", Name = "Max Walsh" }
};
var sortedResults = a.Join(res, s => s, e => e.IndividualProfileId, (s, e) => e);
It's impossible to have the datasource get the results directly in the right order, unless you're willing to write some dedicated SQL stored procedure. The problem is that you'd have to tell the database the contents of a. Using LINQ this can only be done via Contains. And that doesn't guarantee any order in the result set.

Turn the list(A), which you stated is a string, into an actual list. For example, you could use listAsString.Split(",") and then remove the 's from each element. I’ll assume the finished list is called list.
Query the database to retrieve the rows that you need, for example:
var data = db.Table.Where(row => list.Contains(row.IndividualProfileId));
From the data returned, create a dictionary keyed by the IndividualProfileId, for example:
var dic = data.ToDictionary(e => e.IndividualProfileId);
Iterate through the list and retrieve the dictionary entry for each item:
var results = list.Select(item => dic[item]).ToList();
Now results will have the records in the same order that the IDs were in list.

My code is very inefficient for this simple Linq usage

I have the following method that is supposed to parse information from an XML response and return a collection of users.
I've opted into creating a Friend class and returning a List<Friend> to the calling method.
Here's what I have so far, but I noticed that the ids.ToList().Count method parses every single id element to a List, then does it again in the for conditional. It's just super ineffective.
public List<Friend> FindFriends()
{
List<Friend> friendList = new List<Friend>();
var friends = doc.Element("ipb").Element("profile").Element("friends").Elements("user");
var ids = from fr in friends
select fr.Element("id").Value;
var names = from fr in friends
select fr.Element("name").Value;
var urls = from fr in friends
select fr.Element("url").Value;
var photos = from fr in friends
select fr.Element("photo").Value;
if (ids.ToList().Count > 0)
{
for (int i = 0; i < ids.ToList().Count; i++)
{
Friend buddy = new Friend();
buddy.ID = ids.ToList()[i];
buddy.Name = names.ToList()[i];
buddy.URL = urls.ToList()[i];
buddy.Photo = photos.ToList()[i];
friendList.Add(buddy);
}
}
return friendList;
}

First question - do you have to return a List<Friend>? Can you return an IEnumerable<Friend> instead? If so, performance gets a lot better:
IEnumerable<Friend> FindFriends()
{
return doc.Descendants("user").Select(user => new Friend {
ID = user.Element("id").Value,
Name = user.Element("name").Value,
Url = user.Element("url").Value,
Photo = user.Element("photo").Value
});
}
Rather than actually creating new buckets and stuffing values into them, this creates a projection, or a new object that simply contains all of the logic for how to create the new Friend objects without actually creating them. They get created when the caller eventually starts to foreach over the IEnumerable. This is called "deferred execution".
This also makes one assumption - All the <user> nodes in your XML fragment are friends. If that isn't true, the first part of the XML selection might need to be a little more complex.
And as #anon points out, even if you do need to return a List<Friend> for some reason not obvious from the information you've provided, you can just call .ToList() at the end of the return statement. This will just execute the projection I described above straight into a new bucket, so you only ever create one.

Why do you need the separate ids/names/urls/photos variables? Combine it all. You can eliminate the ToList() call if you don't need a List.
List<Friend> friendList = (from f in doc.Element("ipb").Element("profile").Element("friends").Elements("user")
select new Friend() {
ID = f.Element("id").Value,
Name = f.Element("name").Value,
URL = f.Element("url").Value,
Photo = f.Element("photo").Value
}).ToList();
return friendList;