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AutoMapper
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Description

AutoMapper support for updating existing collections by equivalency

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AutoMapper

AutoMapper.Collection

Adds ability to map collections to existing collections without re-creating the collection object.

Will Add/Update/Delete items from a preexisting collection object based on user defined equivalency between the collection's generic item type from the source collection and the destination collection.

NuGet

How to add to AutoMapper?

Call AddCollectionMappers when configuring

Mapper.Initialize(cfg =>
{
    cfg.AddCollectionMappers();
    // Configuration code
});
Will add new IObjectMapper objects into the master mapping list.

Adding equivalency between two classes

Adding equivalence to objects is done with EqualityComparison extended from the IMappingExpression class.

cfg.CreateMap().EqualityComparison((odto, o) => odto.ID == o.ID);
Mapping OrderDTO back to Order will compare Order items list based on if their ID's match
Mapper.Map,List>(orderDtos, orders);
If ID's match, then AutoMapper will map OrderDTO to Order

If OrderDTO exists and Order doesn't, then AutoMapper will add a new Order mappeed from OrderDTO to the collection

If Order exists and OrderDTO doesn't, then AutoMapper will remove Order from collection

Why update collection? Just recreate it

ORMs don't like setting the collection, so you need to add and remove from preexisting one.

This automates the process by just specifying what is equal to each other.

Can it just figure out the ID equivalency for me in Entity Framework?

Automapper.Collection.EntityFramework
or
Automapper.Collection.EntityFrameworkCore
can do that for you.
Mapper.Initialize(cfg =>
{
    cfg.AddCollectionMappers();
    cfg.SetGeneratePropertyMaps>();
    // Configuration code
});

User defined equality expressions will overwrite primary key expressions.

What about comparing to a single existing Entity for updating?

Automapper.Collection.EntityFramework does that as well through extension method from of DbSet.

Translate equality between dto and EF object to an expression of just the EF using the dto's values as constants.

dbContext.Orders.Persist().InsertOrUpdate(newOrderDto);
dbContext.Orders.Persist().InsertOrUpdate(existingOrderDto);
dbContext.Orders.Persist().Remove(deletedOrderDto);
dbContext.SubmitChanges();
Note: This is done by converting the OrderDTO to Expression> and using that to find matching type in the database. You can also map objects to expressions as well.

Persist doesn't call submit changes automatically

Where can I get it?

First, install NuGet. Then, install AutoMapper.Collection from the package manager console:

PM> Install-Package AutoMapper.Collection

Additional packages

AutoMapper Collection for Entity Framework

PM> Install-Package AutoMapper.Collection.EntityFramework

AutoMapper Collection for Entity Framework Core

PM> Install-Package AutoMapper.Collection.EntityFrameworkCore

AutoMapper Collection for LinqToSQL

PM> Install-Package AutoMapper.Collection.LinqToSQL

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