How to write Repository method for .ThenInclude in EF Core 2

Question

I\'m trying to write a repository method for Entity Framework Core 2.0 that can handle returning child collections of properties using .ThenInclude, but I\'m having trouble

Answer 1

I had the same issue since EF Core doesn't support lazy loading but i tried to get workaround in the following way:

First create an attribute class to mark our desired navigation properties from other properties of a given class.

[AttributeUsage(AttributeTargets.Property, Inherited = false)]
public class NavigationPropertyAttribute : Attribute
{
    public NavigationPropertyAttribute()
    {
    }
}

Extension methods to filter out navigation properties and apply Include/ThenInclude using string based Eager loading.

public static class DbContextHelper
{

    public static Func, IQueryable> GetNavigations() where T : BaseEntity
    {
        var type = typeof(T);
        var navigationProperties = new List();

        //get navigation properties
        GetNavigationProperties(type, type, string.Empty, navigationProperties);

        Func, IQueryable> includes = ( query => {
                    return  navigationProperties.Aggregate(query, (current, inc) => current.Include(inc));   
            });

        return includes;
    }

    private static void GetNavigationProperties(Type baseType, Type type, string parentPropertyName, IList accumulator)
    {
        //get navigation properties
        var properties = type.GetProperties();
        var navigationPropertyInfoList = properties.Where(prop => prop.IsDefined(typeof(NavigationPropertyAttribute)));

        foreach (PropertyInfo prop in navigationPropertyInfoList)
        {
            var propertyType = prop.PropertyType;
            var elementType = propertyType.GetTypeInfo().IsGenericType ? propertyType.GetGenericArguments()[0] : propertyType;

            //Prepare navigation property in {parentPropertyName}.{propertyName} format and push into accumulator
            var properyName = string.Format("{0}{1}{2}", parentPropertyName, string.IsNullOrEmpty(parentPropertyName) ? string.Empty : ".", prop.Name);
            accumulator.Add(properyName);

            //Skip recursion of propert has JsonIgnore attribute or current property type is the same as baseType
            var isJsonIgnored = prop.IsDefined(typeof(JsonIgnoreAttribute));
            if(!isJsonIgnored && elementType != baseType){
                GetNavigationProperties(baseType, elementType, properyName, accumulator);
            }
        }
    }
}

Sample POCO classes implementing NavigationPropertyAttribute

public class A : BaseEntity{
  public string Prop{ get; set; }
}

public class B : BaseEntity{
   [NavigationProperty]
   public virtual A A{ get; set; }
}

public class C : BaseEntity{
   [NavigationProperty]
   public virtual B B{ get; set; }
}

Usage in Repository

public async Task GetAsync(Expression> predicate)
{
    Func, IQueryable> includes = DbContextHelper.GetNavigations();
    IQueryable query = _context.Set();
    if (includes != null)
    {
        query = includes(query);
    }

    var entity = await query.FirstOrDefaultAsync(predicate);
    return entity;
}

Json result for sample class C would be:

{
  "B" : {
        "A" : {
              "Prop" : "SOME_VALUE"
            }
      }
}