Partial updates of entities with collections

[MichaelM223]

I was struggling with updating some entities with collections that have their collection set to null when relationships have not been resolved yet. EF Core skips null collections for when checking for relationship modifications.
EntityCollectionTypeMapper and MergeEntityCollectionTypeMapper' however map null` values to an empty collection.

With non-collection members I can agree you can't differ from removal or not set when a null value is passed. With collections though I think a empty collection indicates deletion of all relationships and null is clearly 'not-set' in my opinion.

The changes below work perfectly in my case but maybe I'm forgetting something?

Would it be ok if I submitted a pull request for two small changes in EntityCollectionTypeMapper and `MergeEntityCollectionTypeMapper'?
After line 31 I would add:

if (source == null)
{
    return target;
}

Detached-Mapper/src/Detached.Mappers/TypeMappers/Entity/Collection/EntityCollectionTypeMapper.cs

Lines 30 to 33 in 66bd475

	public override TTarget Map(TSource source, TTarget target, IMapContext context)
	{
	TTarget result = _construct(context);
	Dictionary<TKey, TTargetItem> table = new Dictionary<TKey, TTargetItem>();

Detached-Mapper/src/Detached.Mappers/TypeMappers/Entity/Collection/MergeEntityCollectionTypeMapper.cs

Lines 30 to 33 in 66bd475

	public override TTarget Map(TSource source, TTarget target, IMapContext context)
	{
	TTarget result = _construct(context);
	Dictionary<TKey, TTargetItem> table = new Dictionary<TKey, TTargetItem>();

[leonardoporro]

Makes sense, but I would do it configurable just in case it breaks scenarios for other few people who is using it

[MichaelM223]

Good idea, I'll try to figure out something configurable and make a pull request

[MichaelM223]

Took a while but finally had time create a pull request (#100)

[leonardoporro]

Thank you!
Sorry, I will have time to check on it this weekend. i.e.: tomorrow night or on Saturday

[leonardoporro]

I manually added the feature and it's called MapParameters.CompositeCollectionBehavior
And the values are Append and Merge.. where Append ignore nulls and Merge will delete the DB.
Does this make sense?

[leonardoporro]

It's 4am here.. so maybe it doesn't