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Entity Framework Core Generating SQL With Ambiguous Column Names

I am using Entity Framework Core 2.2.6. I'm going to try and make this question concise and apologies in advance if it ends up being a wall of text.
The error I am seeing is an ambiguous column name in the SQL Entity Framework Core generates.
So my situation is this: I have two entities with a many-to-one relationship. The "parent" entity implements
an interface that has a property that is of type IChildEntity. Here are the interfaces:
public interface IParentEntity
{
IChildEntity Child { get; set; }
string Prop1 { get; set; }
string Prop2 { get; set; }
}
public interface IChildEntity
{
string ChildProp1 { get; set; }
string ChildProp2 { get; set; }
}
I am using ef core's fluent api and in order to set up the relationship between parent and child
I am using a concrete type of ChildEntity and defining a IChildEntity property to conform to the
interface and just passing things through to the concrete type:
public class ChildEntity : IChildEntity
{
public long ID {get; set;}
public string ChildProp1 { get; set; }
public string ChildProp2 { get; set; }
}
public class ParentEntity : IParentEntity
{
public long ID { get; set; }
public string Prop1 { get; set; }
public string Prop2 { get; set; }
public long ChildID { get; set; }
// Navigation property so EF Core can create the relationship
public ChildEntity MappedChild { get; private set; }
// this is to adhere to the interface
// just pass this through to the backing concrete instance
[NotMapped]
public IChildEntity Child
{
get => MappedChild;
set => MappedChild = (ChildEntity)value;
}
}
Then in OnModelCreating I set up the relationship like so:
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<ParentEntity>()
.HasOne(e => e.MappedChild)
.WithMany()
.HasForeignKey(e => e.ChildID);
}
This works and the relationship gets set up as expected, however I am finding when I do a query it can generate
some SQL that can result in an ambigous column error in some database engines. Here is the example query:
MyContext.ParentEntity
.Include(p => p.MappedChild)
.Where(p => p.Prop1.Equals("somestring")
.FirstOrDefault()
The SQL that gets generated is similar to:
SELECT p."ID", p."ChildID", p."Prop1", p."Prop1", "p.MappedChild"."ID", "p.MappedChild"."ChildProp1", "p.MappedChild"."ChildProp2"
FROM "ParentEntity" AS p
INNER JOIN "ChildEntity" AS "p.MappedChild" ON p."ChildID" = "p.MappedChild"."ID"
WHERE p."Prop1" = 'somestring'
ORDER BY "p.MappedChild"."ID"
LIMIT 1
The problem here is we are selecting two columns with the name ID and not aliasing. Some databases will be ok with this
but some will not. A work around I can do for this is to do two separate queries to get the entity and the child entity:
var parent = MyContext.ParentEntity
.Where(p => p.Prop1.Equals("somestring")
.FirstOrDefault()
MyContext.Entry(parent).Reference(p => s.MappedChild).Load();
But this is less than ideal since it does multiple queries and is a bit less elegant than just using Include()
Because this seems like such a common use case and I couldn't find any bug reports against EF Core for this type of
behavior it is my suspicion that I am doing something wrong here that is resulting in EFCore not aliasing column names
for this type of query. I was thinking it could be the bit of trickery I have to do to ensure my entity implements it's interface
(this is something I can't due to constraints in the codebase and other integrations) but the more I look at it the less likely that
seems to me since we are directly dealing with the "mapped" property in EF related code and it's completely unaware of the interface.
My questions are - can anyone see something in my implementation that would cause this? Could anyone
suggest a better workaround than what I have here? Any advice here would be appreciated. Thanks much.

This is an old Entity framework bug with the Oracle company products bug including the MySQL database and Oracle database (12.1 and older).
I see the
ORA-00918: column ambiguously defined
error mostly when:
Selecting one entity with including parent entity.
Selecting one entity with value object own one command
This error appears when using Find, First, FirstOrDefault, Last, Single and all single entity selector commands.
I tested many solutions and check generated sql statement to find out a very unique way without any performance overhead:
// This the way of getting one entity from oracle 12.1 without throwing Oracle exception => ORA-00918: column ambiguously defined without any extra overhead
var entities = await dbSet.Where(x => x.Id == id).Take(1).ToListAsync();
var entity = entities.FirstOrDefault();
Another Sample:
var entities = await dbSet.OrderByDescending(x => x.Id).Take(1).ToListAsync();
var entity = entities.FirstOrDefault();
At the end of your IQueryable Linq add Take(1) and get all with .ToList() or .ToListAsync() to execute the statement and fetch a list with one record. Then use Enumerable Single Entity Selector to change the list to an entity.
That’s all.

Joining Entity Framework tables using Linq when IDs do not exist in both tables

Below is a class I have used to generate a table in my database using Entity Framework. I'd like to be able to link this table to another table, Property. However, the way my code is set up there is not an Id column in the Instruction table, there is a Property property within the class, which then generates a PropertyId column in the actual database, but since the Property property is not an Id I am unable to using Linq to join these tables.
Instruction table
[Table("Instruction")]
public class Instruction
{
[Key]
public int Id { get; set; }
public InstructionTypes InstructionType { get; set; }
public Property Property { get; set; } //Generates the EF property FK, but is not an ID so therefore cannot be used in linq.
}
Property table
[Table("Property")]
public partial class Property
{
[Key]
public int Id { get; set; }
public Address Correspondence { get; set; }
}
Join Query
var instruction =
from instructions in _context.Instructions
join properties in _context.Properties on instructions.Property equals properties.Id
where ...
The above query gives a compiler error of: `The type of one of the expressions in the join clause is incorrect.
This error is being generated as I'm attempting to use a property object to join with a propertyId.
How can I alter this query so that I am able to join these two tables?

In 99% of all cases, you do not want to use the join operator. Entity Framework automatically generates SQL JOINS for you when you are using Navigation Properties.
var instruction = await _context.Instructions.Where(i => i.Property...).FirstOrDefaultAsync().ConfigureAwait(false);
Note, that depending on whether you are using EF6 or EF Core or with different configuration, Lazy Loading may be disabled (if not, I strongly encourage you to disable it as it is a massive performance bottleneck).
So you have to use the Include Method to eagerly load the related entity.
var instruction = await _context.Instructions.Include(i => i.Property).Where(i => i.Property...).FirstOrDefaultAsync().ConfigureAwait(false);
But before doing this, think if you really need the Instruction. If not, your code could become:
var property = await _context.Properties.Where(p => p.Instructions.Any(i => ...)).FirstOrDefaultAsync().ConfigureAwait(false);
Please note that you have to extend your Property class for this to work to have a back-reference
public partial class Property
{
// No need for the Key attribute, as this is convention
public int Id { get; set; }
public Address Correspondence { get; set; }
public int CorrespondenceId { get; set; } // Not needed in this scenario, but good practice
public ICollection<Instruction> Instructions { get; } = new HashSet<Instruction>();
}

You seems to be a newcomer to linq. As such you are still thinking as if you still are in an sql world.
With linq to entities, the use of join is the exception. SQL join are generated silently by EF using the navigation properties.
So your query can be:
var instruction =
from instruction in _context.Instructions
where instruction.Porperty.Correspondence.Contains("abc");
then you can access
instruction.First().Property.Correspondence
As a good practice you can delclare the foreign keys as class members and use the fluent API to bind them.
To test you can use the following code,
//assuming that Instructions is a DbSet<Instruction>
using (var context = new MyContext() ) {
context.Instructions.Add(
new instruction {
Property = new Property {
Correspondence = new Address {}
}
});
}
using (var context = new MyContext() ) {
var c = context.Instructions.First();
console.WriteLine($"{c.Id}, {c?.Property.Id}, {c?.Property?.Correspondence.Id}");
});

The specified type member is not supported in LINQ to Entities. entity members, and entity navigation

public partial class User : IUser
{
public long ID {get; set;}
public BaseUser BaseUser
{
get
{
var context = new Factory().Create<ContextDB>();
return context.Users.Find(this.ID);
}
}
}
and
var result = _Context.Employees.Where(t => t.User.BaseUser.UserName.ToLower().Trim().Contains(searchKey));
Here I am getting an exception:
The specified type member 'BaseUser' is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation properties are supported.
Any solution to this?

You have written Linq query. And you are working with Entity Framework. So, you are using Linq-To-EntityFramework.
.Net will translate your Linq query to database query based on the type of the RDMS which you are using.
For example, let's take this code:
context.Users.Select(x => x.Id == 5);
This will be translated to:
select * from User where Id=5;
So, it means that .net will throw exception if it couldn't translate it to the database query. For example, your query. You have created property in your class. And you are beleiving that it will be translated to the database query? How? There is no way! This is the reason of the exception.
Also, your BaseUser property seems unusual to me. BaseUser will be same with User if you have configured everything right.

Linq to entities DbQuery object is failing hard

I wonder if anyone can shed some light on what may be happening here. I'm using C#, MVC, with entity framework.
So I run these two lines of code:
var booboo = _context.AppItems.Where(ai => ai.id == 101);
var sql = booboo.ToString();
And I get some strange behavior. The booboo.ToString() method hangs, thus failing. Nothing about the booboo DbQuery object works properly in fact.
I'm having similar problems all over the code with my AppItem entity (AppItems is DbSet as you might guess). Entity Framework appears to be unable to construct a query for the AppItem entity.
Edit:
I wasn't patient enough! After leaving it for a very long time, I do get the following exception:
"Message=Internal error: An expression services limit has been reached. Please look for potentially complex expressions in your query, and try to simplify them."
Interestingly that's a Sql.Client exception, which I wasn't expecting.
Here's what the AppItem class looks like:
public class AppItem : Domain.Item
{
public int? UserProfileId { get; set; }
public virtual UserProfile UpdatedByUser { get; set; }
[MaxLength(50)]
public String Type { get; set;}
public DateTime UpdatedDate { get; set;}
// flags
public virtual ICollection<ItemFlag> Flags { get; set; }
// actions
public virtual ICollection<ItemAction> Actions { get; set; }
// notes
public virtual ICollection<Note> Notes { get; set; }
}
Domain Item contains a primary key field (id) and a few other fields.
The Note / ItemAction / ItemFlag Classes there all inherit from AppItem, so perhaps some sort of circular referencing is to blame?
Other items can be queried just fine. For example, I have numerous classes that inherit from AppItem (like ItemFlag, ItemAction and Note) and I can query all of these just fine.
So, where Members is DbSet and Member inherits from AppItem:
var foofoo = _context.Members.Where(ai => ai.id = 101);
var sql = foofoo.ToString();
This Works fine; foofoo.ToString() returns the constructed SQL and everything appears to be in order.
It seems really bizarre to me, there's no error message or anything, the application just hangs when it tries to query AppItems. The table exists in the database, but that doesn't matter because we aren't getting as far as querying the database, we are failing to construct a query in the first place.
Thanks in advance for any help.

I found what the problem was.
I'm using Table-per-Type for inheritance. With AppItem being a base type, the SQL query it generates for querying it is huge (several thousand lines long in this case) and causes problems.
So basically, you need to avoid querying on base types that have more than a few types inheriting from them when using Table-per-Type.

I am assuming that your query is meant to return 1 item.
Add .FirstOrDefault(); onto the end of your query to only return one item (your current query returns an IQueriable of type AppItems)
The Entity Framework does not execute the query until needed, so in your code it will execute the query when the .ToString() method is called.
Try this:
var booboo = _context.AppItems.Where(ai => ai.id == 101).FirstOrDefault();
if (booboo != null)
{
var sql = booboo.ToString();
//etc
}

Dynamic Expression for Ordering Child Collections with Entity Framework

I am new to EF. I am trying to get Entity Framework 4.2 to do a sort by a calculated property (not mapped).
Here is what my entity look like:
public class Site : Entity
{
public Site()
{
Equipments = new HashSet<Equipment>();
Forecasts = new HashSet<Forecast>();
}
[StringLength(8)]
public string Number { get; set; }
[StringLength(50)]
public string EquipmentShortCLLI { get; set; }
[StringLength(50)]
public string Location { get; set; }
public virtual Central Central { get; set; }
public virtual ICollection<Equipment> Equipments { get; set; }
public virtual ICollection<Forecast> Forecasts { get; set; }
#region Calculated Items
public bool IsEmbargo {
get { return Equipments.Count > 0 && Equipments.SelectMany(x => x.EquipmentDetails).Any(e => e.IsEmbargo); }
}
//...
public int PortsCapacity
{
get
{
return Equipments.Count > 0
? Equipments.SelectMany(x => x.Slots).Sum(x => x.PortsCapacity)
: 0;
}
}
#endregion
//...
By trying to order using any of my readonly properties I am getting the exception:
The specified type member 'PortsCapacity' is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation properties are supported.
Which makes sense because EF is trying to build an sql orderby with a field that does not exist in the database (my understanding..).
Now, by using some dynamic linq code I was able to make this work for my many-to-one columns by passing "Central.SomeField" (as opposed to making a ReadOnly Property that returns Central.SomeField).
I.E.:
query.OrderBy("Central.SomeField");
However, I still face the same issue when it comes to a collection of items (Equipments). I am trying to make this as dynamic as possible by using a string coming from the client side and avoiding a long switch case, but at this point I will accept any ideas, so long as the sorting happens on the database side.
Edit 1:
Following what Ladislav Mrnka says, how would one execute an OrderBy clause on one-to-many child items using lambdas or expression?

I don't think that Dynamic Linq is capable of this. You need a real Linq subquery to compute aggregations on Equipements so it will simply not work. If the user selects ordering by IsEmbargo or PortsCapacity you must have some switch / if block to handle this case by appending special part of the query - no other way.