Rewrite all column references to one or more aliases to a new single alias
Inheritance: DbExpressionVisitor
        protected override Expression VisitProjection(ProjectionExpression proj)
        {
            SelectExpression save = this.currentSelect;

            this.currentSelect = proj.Select;
            try
            {
                if (!this.isTopLevel)
                {
                    if (this.CanJoinOnClient(this.currentSelect))
                    {
                        // make a query that combines all the constraints from the outer queries into a single select
                        SelectExpression newOuterSelect = (SelectExpression)QueryDuplicator.Duplicate(save);

                        // remap any references to the outer select to the new alias;
                        SelectExpression newInnerSelect = (SelectExpression)ColumnMapper.Map(proj.Select, newOuterSelect.Alias, save.Alias);
                        // add outer-join test
                        ProjectionExpression newInnerProjection = this.dbExpressionBuilder.AddOuterJoinTest(new ProjectionExpression(newInnerSelect, proj.Projector));
                        newInnerSelect = newInnerProjection.Select;
                        Expression newProjector = newInnerProjection.Projector;

                        TableAlias newAlias = new TableAlias();
                        var        pc       = ColumnProjector.ProjectColumns(newProjector, null, newAlias, newOuterSelect.Alias, newInnerSelect.Alias);

                        JoinExpression   join         = new JoinExpression(JoinType.OuterApply, newOuterSelect, newInnerSelect, null);
                        SelectExpression joinedSelect = new SelectExpression(newAlias, pc.Columns, join, null, null, null, proj.IsSingleton, null, null, false);

                        // apply client-join treatment recursively
                        this.currentSelect = joinedSelect;
                        newProjector       = this.Visit(pc.Projector);

                        // compute keys (this only works if join condition was a single column comparison)
                        List <Expression> outerKeys = new List <Expression>();
                        List <Expression> innerKeys = new List <Expression>();
                        if (this.GetEquiJoinKeyExpressions(newInnerSelect.Where, newOuterSelect.Alias, outerKeys, innerKeys))
                        {
                            // outerKey needs to refer to the outer-scope's alias
                            var outerKey = outerKeys.Select(k => ColumnMapper.Map(k, save.Alias, newOuterSelect.Alias));
                            // innerKey needs to refer to the new alias for the select with the new join
                            var innerKey = innerKeys.Select(k => ColumnMapper.Map(k, joinedSelect.Alias, ((ColumnExpression)k).Alias));
                            ProjectionExpression newProjection = new ProjectionExpression(joinedSelect, newProjector, proj.Aggregator);
                            return(new ClientJoinExpression(newProjection, outerKey, innerKey));
                        }
                    }
                    else
                    {
                        bool saveJoin = this.canJoinOnClient;
                        this.canJoinOnClient = false;
                        var result = base.VisitProjection(proj);
                        this.canJoinOnClient = saveJoin;
                        return(result);
                    }
                }
                else
                {
                    this.isTopLevel = false;
                }
                return(base.VisitProjection(proj));
            }
            finally
            {
                this.currentSelect = save;
            }
        }