r data.table圍繞ad-hoc連接的函數包裝器（在鏈中有聚合）

Question

[data.table_1.9.6] 問題的背景是我試圖在星型模式中構建類似olap的查詢功能像數據佈局，即一個大的事實表和幾個元表。我建立一個函數來包裹data.table聯接，然後聚集在鏈這樣：

# dummy data 
dt1 = data.table(id = 1:5, x=letters[1:5], a=11:15, b=21:25) 
dt2 = data.table(k=11:15, z=letters[11:15]) 

# standard data.table query with ad-hoc key -> works fine 
dt1[dt2, c("z") := .(i.z), with = F, 
    on = c(a="k")][, .(m = sum(a, na.rm = T), 
        count = .N), by = c("z")] 

# wrapper function with setkey -> works fine 
agg_foo <- function(x, meta_tbl, x_key, meta_key, agg_var) { 
    setkeyv(x, x_key) 
    setkeyv(meta_tbl, meta_key) 
    x[meta_tbl, (agg_var) := get(agg_var)][,.(a_sum = sum(a, na.rm=T), 
              count = .N), 
             by = c(agg_var)] 
    x[, (agg_var) := .(NULL)] 
    } 

# call function (works fine) 
agg_foo(x=dt1, meta_tbl=dt2, x_key="a", meta_key="k",agg_var="z") 

# wrapper function with ad-hoc key -> does not work 
agg_foo_ad_hoc <- function(x, meta_tbl, x_key, meta_key, agg_var) { 
    x[meta_tbl, (agg_var) := get(agg_var), 
    on = c(x_key = meta_key)][,.(a_sum = sum(a, na.rm=T), 
           count = .N), by = c(agg_var)] 
    x[, (agg_var) := .(NULL)] 
    } 

# call function (causes error) 
agg_foo_ad_hoc(x=dt1, meta_tbl=dt2, x_key="a", meta_key="k",agg_var="z") 

Error in forderv(x, by = rightcols) : 
    'by' value -2147483648 out of range [1,4] 

我的猜測是，我必須提供「的」參數的即席以不同的方式。我嘗試了= c（get（x_key）= meta_key），但他抱怨意外的括號。我可以使用setkey版本的函數，但我不知道這是否有效，因爲該函數可以在不同的元表上工作，這取決於使用哪個屬性來聚合，並因此不斷重新設置密鑰。或者是setkey總是首選？實際的事實表（這裏是x）有> 30萬行。

Answer 1

所有你需要做的就是構建一個帶有正確標籤的矢量。下面是這樣做的一種方式：

agg_foo_ad_hoc <- function(x, meta_tbl, x_key, meta_key, agg_var) { 
    x[meta_tbl, (agg_var) := get(agg_var), 
    on = setNames(meta_key, x_key)][,.(a_sum = sum(a, na.rm=T), 
             count = .N), by = c(agg_var)] 
    x[, (agg_var) := .(NULL)] 
}