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Showing content from https://github.com/AutoMapper/AutoMapper.Collection below:

AutoMapper/AutoMapper.Collection: AutoMapper support for updating existing collections by equivalency

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.

Call AddCollectionMappers when configuring

Mapper.Initialize(cfg =>
{
    cfg.AddCollectionMappers();
    // Configuration code
});

Will add new IObjectMapper objects into the master mapping list.

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

cfg.CreateMap<OrderItemDTO, OrderItem>().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<OrderDTO>,List<Order>>(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 mapped from OrderDTO to the collection

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

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.

Automapper.Collection.EntityFramework or Automapper.Collection.EntityFrameworkCore can do that for you.

Mapper.Initialize(cfg =>
{
    cfg.AddCollectionMappers();
// entity framework
    cfg.SetGeneratePropertyMaps<GenerateEntityFrameworkPrimaryKeyPropertyMaps<DB>>();
// entity framework core
cfg.SetGeneratePropertyMaps<GenerateEntityFrameworkCorePrimaryKeyPropertyMaps<DB>>();
    // Configuration code
});

User defined equality expressions will overwrite primary key expressions.

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<OrderDTO>(newOrderDto);
dbContext.Orders.Persist().InsertOrUpdate<OrderDTO>(existingOrderDto);
dbContext.Orders.Persist().Remove<OrderDTO>(deletedOrderDto);
dbContext.SubmitChanges();

Note: This is done by converting the OrderDTO to Expression<Func<Order,bool>> 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

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

PM> Install-Package AutoMapper.Collection

PM> Install-Package AutoMapper.Collection.EntityFramework

PM> Install-Package AutoMapper.Collection.EntityFrameworkCore

PM> Install-Package AutoMapper.Collection.LinqToSQL

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