public NumberExpression<Long> count() {
        if (count == null) {
            count = Expressions.numberOperation(Long.class, Ops.AggOps.COUNT_AGG, mixin);
        }
        return count;
    }
Get the {@code count(this)} expression

@return count(this)
querydsl-core/src/main/java/com/querydsl/core/types/dsl/SimpleExpression.java