如何在组中加入和获取行数 - 所有这些都在一个data.table语句中？

Question

我尝试做

library(data.table)

customers <- data.table( id=c(1,2,3,4),                               # id of customers
                         name=c("Frank","Hans","Peter","Markus"),
                         age=c(22,34,64,19))
purchases <- data.table( id=c(1,2,3,4,5,6),                           # id of purchases
                         customer_id=c(1,2,4,2,1,5), 
                         name=c("CD","hairdryer","book","glas","paper","chair"),
                         product_type=c("home","home","home","home","office","office")
                         )

purchases[customers, .N, by=product_type, on=c(customer_id = "id")]
purchases[customers, mean(age), by=product_type, on=c(customer_id = "id")]

并期望第一次加入

   product_type N
1:         home 4
2:           NA 1
3:       office 1

但得到了

   product_type N
1:         home 4
2:           NA 1
3:         home 1

第二个命令应该进行连接，然后在product_type上进行分组并计算每个组的年龄平均值。但是我收到age未找到的错误。

purchases[customers, mean(age), by=.EACHI, on=c(customer_id = "id")]

顺便说一下，

工作得非常出色。