EntityFramework Include and possibly join? - mysql

Question

I have the following table structure as shown in the picture. (see: Table structure). Both tables ("Batches" and "Methods") reference to a "Project" table.
When I now create a new Project I would like to get all childs created as well.
Doing so I did the follwoing:
_dbContext.Projects.Where(x => x.Id == prjId)
.Include(x => x.Batches)
.Include(x => x.Batches.Select(y => y.Measurements))
.Include(x => x.Methods).AsNoTracking().FirstOrDefault();
Now the problem is the following:
New Batch and Method instances are created - thus they get a new ID(PK). The referenced Project_Id (FK) is set correct. But in my new Measurement instance only the Batch_Id(FK) is set correct and the Method_Id remains unchanged (has the old value) (see: result).
What I need is that the Measurements.Mehtod_Id is set from the Methods table. Is there any suitable solution for that?
My entities look like the following
public class Project
{
[Key]
public long Id { get; set; }
public string Name { get; set; }
public bool IsActive { get; set; }
public virtual List<Batch> Batches { get; set; }
public virtual List<Method> Methods { get; set; }
}
public class Batch : BaseObject
{
public Batch()
{
BatchFiles = new List<FileAttachment>();
Measurements = new List<Measurement>();
}
public long Id { get; protected set; }
public long Project_Id { get; set; }
public virtual Project Project { get; set; }
public virtual List<Measurement> Measurements { get; set; }
}
public class Method : BaseObject
{
public Method()
{
Parameters = new List<Parameter>();
}
public long Id { get; protected set; }
public long Project_Id { get; set; }
public virtual Project Project { get; set; }
public virtual List<Measurement> Measurements { get; set; }
}
public class Measurement
{
public int Id { get; protected set; }
[ForeignKey("Batch")]
public long? Batch_Id { get; set; }
[Required]
public virtual Batch Batch { get; set; }
[ForeignKey("Method")]
public long? Method_Id { get; set; }
public virtual Method Method { get; set; }
}
// creation code (just a copy with new IDs for all childs)
Project newProjectVersion = _dbContext.Projects.Where(x => x.Id == prjId)
.Include(x => x.Batches)
.Include(x => x.Batches.Select(y => y.Measurements))
.Include(x => x.Methods)
.AsNoTracking().FirstOrDefault();
_dbContext.Projects.Add(newProjectVersion);
_dbContext.SaveChanges();
Thanks for any help!

Answer 1

The first problem is that your Select statement doesn't connect Measurements to Methods because of the AsNoTracking() addition. Only Projects and Methods are connected because they are explicitly Included off of the Project entity. The Measurements have a Method_id but this is value is not accompanied by a Method in their Method property. You could check that in the debugger if you walk through the object graph (with lazy loading disabled though!). Because of this, when all entities will be Add-ed to the context, EF won't notice that measurements receive new methods.
You could get tempted to fix that by Include-ing Measurement.Method as well:
...
.Include(x => x.Batches.Select(y => y.Measurements.Select(m => m.Method)))
...
Now you'll see that Measurement.Method will be populated everywhere in the object graph.
However, there's a gotcha here. When using AsNoTracking, EF6 doesn't keep track of entities it materialized (duh). This means that for each Measurement it creates a new Method instance, even if an identical Method (by id) was materialized before for another Measurement. (And in this case it will always materialize duplicates, because you already include Project.Methods.)
That's why you can't do this in the quick way with AsNoTracking and Add using one context instance. You'll get an error that EF tries to attach duplicate entities.
You must build the object graph using one context, with tracking, so EF will not materialize duplicates. Then you must Add this object graph to a new context. Which will look like this:
Project project;
using(var db = new MyContext())
{
db.Configuration.ProxyCreationEnabled = false;
project = db.Projects.Where(x => x.Id == prjId)
.Include(x => x.Batches)
.Include(x => x.Batches.Select(y => y.Measurements))
.Include(x => x.Methods).FirstOrDefault();
}
using(var db = new MyContext())
{
db.Projects.Add(project);
db.SaveChages();
}
Three remarks:
Proxy creation is disabled, because you can't attach a proxy to another context without explicitly detaching it first.
No, I didn't forget to include Measurement.Method. All methods are loaded by including them in the Project and now (because of tracking, and assuming that measurement will only have methods of the project they belong to), EF connects them with the Measurements by relationship fixup.
EF-core is smarter here: when adding AsNoTracking it won't track materialized entities, but still, it won't create duplicates either. It seems to have some temporary tracking during the construction of an object graph.

Answer 2

thanks for your answer so far. This works quite fine right now. Unfortunately I noticed that the Measurements entity has another required relationship to a table named 'MeasurementTypes':
[Required]
public virtual MeasurementType MeasurementType { get; set; }
[ForeignKey("MeasurementType")]
public long MeasurementType_Id { get; set; }
In contrast to Batches and Methods these entries must not be copied and the entries already exist in the MeasrementTypes table.
What would be a good way to put the required reference to the Measurements?