Suboptimal breaking change from PR #26 about xodus-dnq HOT 9 CLOSED

The problem is here

fun <S : XdEntity, T : XdEntity> XdQuery<S>.mapDistinct(dbFieldName: String, targetEntityType: XdEntityType<T>): XdQuery<T> {
    return queryEngine.selectDistinct(entityIterable, dbFieldName).asQuery(targetEntityType)
}

The QueryEngine here can return null elements in the resulting iterable. There is no way to predict if the result will contain null or not from the parameters of the function.

Another option would be to create nullable XdQuery<T : XdEntity, E : Entity?>, but it would be a bigger breaking change.

Probably, we should guard the XdQuery#entityIterable somehow.

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Let's take a look how can we do it smarter.

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Another thing here that worries me is that query.size() and query.asSequence().size may differs.

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No nulls - no problems, right? We may change the mapDistinct to ignore null results at all, but I personally dislike this not obvious behavior. Though, we can make it more clear:

• make XdQuery#entityIterable elements not nullable again
• create a mapNotNull method that ignores null results
• make mapDistinct to use mapNotNull
• deprecate mapDistinct method in favor to the mapNotNull
• remove XdQuery#asNullableSequence method

p.s.: mapDistinctNotNull/ mapNotNullDistinct?

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Let's discuss nulls in EntityIterable with @penemue when he will be back. At the current moment seems that nulls may appear only on mapDistinct, flatMapDistinct operations.

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Nulls can be among the results of SelectDistinct or SelectManyDistinct.

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@penemue am I right that result of querying EntityIterable with nulls will be EntityIterable without nulls?

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OK guys. After all discussions I think that we don't need Iterable<Entity?> at all. Yes, it means that it will be impossible to have nulls returned by mapDistinct. I don't feel that we actually need it.

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I'm rolling back all changes related to Iterable<Entity?>. I will add ExcludeNullStaticTypedEntityIterable to mapDistinct / flatMapDistinct.

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