CSHARP-4627: Support Union via $unionWith in LINQ3

[sanych-sun]

No description provided.

[rstam]

Good work. I have some non-trivial refactoring suggestions based on my experience writing LINQ3 translators. Let's meet on Zoom tomorrow to discuss.

[rstam]

I think instead of this change we need a new extension method called Evaluate:

public static object Evaluate(this Expression expression)
{
    if (expression is ConstantExpression constantExpression)
    {
        return constantExpression.Value;
    }
    else
    {
        LambdaExpression lambda = Expression.Lambda(expression);
        Delegate fn = lambda.Compile();
        return fn.DynamicInvoke(null);
    }
}

See below for where it is used.

[sanych-sun]

Done

[rstam]

nit: place in alphabetical order

[sanych-sun]

Done

[rstam]

I would propose the following refactoring:

public static AstPipeline Translate(TranslationContext context, MethodCallExpression expression)
{
    var method = expression.Method;
    var arguments = expression.Arguments;

    if (method.Is(QueryableMethod.Union))
    {
        var firstExpression = arguments[0];
        var pipeline = ExpressionToPipelineTranslator.Translate(context, firstExpression);

        var secondExpression = arguments[1];
        var secondValue = secondExpression.Evaluate();
        if (secondValue is IMongoQueryable secondQueryable)
        {
            var secondProvider = secondQueryable.Provider;
            var secondCollectionName = secondProvider.CollectionNamespace.CollectionName;
            var secondPipelineInputSerializer = secondProvider.PipelineInputSerializer;
            var secondContext = TranslationContext.Create(secondQueryable.Expression, secondPipelineInputSerializer);
            var secondPipeline = ExpressionToPipelineTranslator.Translate(secondContext, secondQueryable.Expression);
            if (secondPipeline.Stages.Count == 0)
            {
                secondPipeline = null;
            }

            pipeline = pipeline.AddStages(
                pipeline.OutputSerializer,
                AstStage.UnionWith(secondCollectionName, secondPipeline));

            return pipeline;
        }
        else if (secondValue is IQueryable)
        {
            throw new ExpressionNotSupportedException(expression, because: "second argument must be IMongoQueryable, not just IQueryable");
        }
        else
        {
            // todo: handle enumerable constant using $documents?
        }
    }

    throw new ExpressionNotSupportedException(expression);
}

Let's discuss over Zoom tomorrow.

[sanych-sun]

Done

[rstam]

AssertStages(stages, "{ $unionWith : 'partners' }");

[sanych-sun]

Done

[rstam]

We normally name the results variable results (same for other test methods).

[sanych-sun]

Done

[rstam]

AssertStages(stages, "{ $unionWith : 'clients' }");

[sanych-sun]

Done

[rstam]

99% there

[rstam]

As a general rule we keep methods in alphabetical order.

[sanych-sun]

Done

[rstam]

Slight change to exception message:

throw new ExpressionNotSupportedException(expression, because: "second argument must be IMongoQueryable");

[sanych-sun]

Done

[rstam]

Delete extra blank line.

[sanych-sun]

Done

[rstam]

Suggested formatting:

var queryable = _firstCollection
    .AsQueryable()
    .Where(c => c.Name.StartsWith("second"))
    .Union(_secondCollection.AsQueryable().Where(p => p.Name.StartsWith("another")));

[sanych-sun]

Done

[rstam]

LGTM

Looks great!