r - Data.table - 多个表的左外连接

Question

假设你有这样的数据

fruits <- data.table(FruitID=c(1,2,3), Fruit=c("Apple", "Banana", "Strawberry"))
colors <- data.table(ColorID=c(1,2,3,4,5), FruitID=c(1,1,1,2,3), Color=c("Red","Yellow","Green","Yellow","Red"))
tastes <- data.table(TasteID=c(1,2,3), FruitID=c(1,1,3), Taste=c("Sweeet", "Sour", "Sweet"))

setkey(fruits, "FruitID")
setkey(colors, "ColorID")
setkey(tastes, "TasteID")

fruits
   FruitID      Fruit
1:       1      Apple
2:       2     Banana
3:       3 Strawberry

colors
   ColorID FruitID  Color
1:       1       1    Red
2:       2       1 Yellow
3:       3       1  Green
4:       4       2 Yellow
5:       5       3    Red

tastes
   TasteID FruitID  Taste
1:       1       1 Sweeet
2:       2       1   Sour
3:       3       3  Sweet

我通常需要对这样的数据执行左外连接。例如，“给我所有的水果和它们的颜色”需要我写(也许有更好的方法？)

setkey(colors, "FruitID")
result <- colors[fruits, allow.cartesian=TRUE]
setkey(colors, "ColorID")

这么简单又频繁的任务，三行代码显得多余，于是写了一个方法 myLeftJoin

myLeftJoin <- function(tbl1, tbl2){
  # Performs a left join using the key in tbl1 (i.e. keeps all rows from tbl1 and only matching rows from tbl2)

  oldkey <- key(tbl2)
  setkeyv(tbl2, key(tbl1))
  result <- tbl2[tbl1, allow.cartesian=TRUE]
  setkeyv(tbl2, oldkey)
  return(result)
}

我可以使用

myLeftJoin(fruits, colors)
   ColorID FruitID  Color      Fruit
1:       1       1    Red      Apple
2:       2       1 Yellow      Apple
3:       3       1  Green      Apple
4:       4       2 Yellow     Banana
5:       5       3    Red Strawberry

我如何扩展这个方法，以便我可以将任意数量的表传递给它并获得所有这些表的链式左外连接？类似 myLeftJoin(tbl1, ...)
例如，我想要 myleftJoin(fruits, colors, tastes) 的结果相当于

setkey(colors, "FruitID")
setkey(tastes, "FruitID")
result <- tastes[colors[fruits, allow.cartesian=TRUE], allow.cartesian=TRUE]
setkey(tastes, "TasteID")
setkey(colors, "ColorID")

result
   TasteID FruitID  Taste ColorID  Color      Fruit
1:       1       1 Sweeet       1    Red      Apple
2:       2       1   Sour       1    Red      Apple
3:       1       1 Sweeet       2 Yellow      Apple
4:       2       1   Sour       2 Yellow      Apple
5:       1       1 Sweeet       3  Green      Apple
6:       2       1   Sour       3  Green      Apple
7:      NA       2     NA       4 Yellow     Banana
8:       3       3  Sweet       5    Red Strawberry

也许我错过了使用 data.table 包中的方法的优雅解决方案？谢谢

(编辑:修正了我的数据中的一个错误)

Answer 1

我刚刚在 data.table, v1.9.5 中提交了一个新功能，我们可以在不设置键的情况下加入(即直接指定要加入的列，而不必先使用 setkey()):

有了这个，这很简单:

require(data.table) # v1.9.5+
fruits[tastes, on="FruitID"][colors, on="FruitID"] # no setkey required
#    FruitID      Fruit TasteID  Taste ColorID  Color
# 1:       1      Apple       1 Sweeet       1    Red
# 2:       1      Apple       2   Sour       1    Red
# 3:       1      Apple       1 Sweeet       2 Yellow
# 4:       1      Apple       2   Sour       2 Yellow
# 5:       1      Apple       1 Sweeet       3  Green
# 6:       1      Apple       2   Sour       3  Green
# 7:       2         NA      NA     NA       4 Yellow
# 8:       3 Strawberry       3  Sweet       5    Red