Getting a single entity property from collection property

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  1. Musashi
    Musashi avatar
    23 posts
    Member since:
    Jan 2014

    Posted 03 Mar 2016 Link to this post

    I have a base class with some properties including a collection of Modification objects. Modification objects just hold modification data, like who modified the record, what was changed, etc.

    I have another class which is like a "summary" version of the base class. Flattens some data out.
    One thing I want it to do is have a Modification property that takes the latest modification from the Modification collection.


    public class BaseEntity
        public int PRIMARYKEY { get; set; }
        public string NAME { get; set; }
        public IList<Modification> Mods { get; set; }

    public class BaseEntitySummary
        public int PRIMARYKEY { get; set; }
        public string NAME { get; set; }
        public Modification LatestMod { get; set; }

    public class Modification
        public int PRIMARYKEY { get; set; }
        public int USERFK { get; set; }
        public int ModNumber { get; set; }
        public DateTime Date { get; set; }

    I work with the BaseEntitySummary class, treating it like an entity. I have a mapping mechanism that maps BaseEntity to BaseEntitySummary, so in the end, what DataAccess sees from this:

    context.BaseEntitySummarys.Where(bes => bes.LatestMod.Date < DateTime.Now)

    is actually

    context.BaseEntitys.Where(be => be.Mods.OrderByDescending(mod => mod.ModNumber).FirstOrDefault().Date < DateTime.Now)
        .Select(be => new BaseEntitySummary
            NAME = be.NAME,
            LatestMod = be.OrderByDescending(mod => mod.ModNumber).FirstOrDefault()


    My problem lies in

    context.BaseEntitys.Where(be => be.Mods.OrderByDescending(mod => mod.ModNumber).FirstOrDefault().Date < DateTime.Now)

    I get a exception stating "Execution of 'System.Linq.Enumerable:FirstOrDefault(IEnumerable`1)' on the database server side currently not implemented."


    Does anyone have any good ideas for making what I want happen? i.e. a property in an entity that takes one object from a collection property?

  2. Doroteya
    Doroteya avatar
    498 posts

    Posted 08 Mar 2016 Link to this post

    Hello Musashi,

    The error you are experiencing is expected. Due the complexity of such a query that is related to the specifics of the scenario, we designed the API to support it with two separate calls to the database. Like this:
    var mod = context.Modifications.OrderByDescending(mod => mod.ModNumber)
        .FirstOrDefault(mod => mod.Date < DateTime.Now);
    var baseEntity = mod.BaseEntity;
    var baseEntitySummary = new baseEntitySummary
        PRIMARYKEY = baseEntity.PRIMARYKEY,
        NAME = baseEntity.NAME,
        LatestMod = mod

    Note that in order to achieve the necessary result, you need the BaseEntity navigation property in the Modification entity.

    I hope this helps.

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