Why do we need ObjectToEquivalencyExpressionByEquivalencyExistingMapper as well

[automapper]

Why do we need ObjectToEquivalencyExpressionByEquivalencyExistingMapper as well

[TylerCarlson1]

It is mostly used for Persist in AM.Collections.EF and AM.Collections.LinqToSQL. What it does is take an object and convert it into an expression that can be used to find its matched object in the database.

So if you need to insert or update you need to know what it is matching on Id or multiple Ids. If you have code

Mapper.Initialize(cfg => 
{
  cfg.AddProfile<CollectionProfile>();
  cfg.CreateMap<Entity,Dto>().EqualityComparison((e, d) => e.Id == d.Id);
}

you can make a map like this

var dto = new Dto {Id = 5};
var findMatchExpression = Mapper.Map<Dto, Expression<Func<Entity,bool>>>(dto); // Returns e => e.Id == 5;
var match = db.Entities.FirstOrDefault(findMatchExpression); // Match will be null or Dto that has an Id of 5

From there you can use the match to insert or update the Entity object. Instead of having to do

var dto = new Dto {Id = 5};
var match = db.Entities.FirstOrDefault(e => e.Id == dto.Id);

Or you can just use

var dto = new Dto {Id = 5};
db.Entities.Persist().InsertOrUpdate<Dto>(dto); // Insert or Update
db.Entities.Persist().Remove<Dto>(dto); // Delete
// call db.SubmitChanges() to commit

If you aren't using this for ORM's to map back to the database, you don't really get any use out of this.
When I made this I made it to be compatible with updating EF/LinqToSQL with as little code as possible.

[TylerCarlson1]

Oh and if you are using it to update EF entities you can add

EquivilentExpressions.GenerateEquality.Add(new GenerateEntityFrameworkPrimaryKeyEquivilentExpressions<MyDBContext>());

then you don't even need EqualityComparison and it's all automatically figured out for you.

[rizi]

Thx for the clarification, I was confused an thought we need the ObjectToEquivalencyExpressionByEquivalencyExistingMapper to make the other mapper (EquivlentExpressionAddRemoveCollectionMapper) work. Now that I know who it works I remember that I did something very similar (https://groups.google.com/d/msg/automapper-users/BwIvt2YsvFo/dE7-HI_kBBEJ) thx in advance and keep the great work!

[TylerCarlson1]

Not sure you know but AutoMapper has the functionality to map expressions now. https://github.com/AutoMapper/AutoMapper/wiki/Expression-Translation-(UseAsDataSource)

And UseAsDataSource is like Persist where it does all those mappings behind the scenes.

[rizi]

I know that AutoMapper supports Expression mapping because I was the first one who make this work and later on Jimmy added it to AutoMapper and also improved it.
Br