r - data.table join and then add all columns from one table to another

Question

My question is essentially the same as this question: data.table join then add columns to existing data.frame without re-copy.

Basically I have a template with keys

Answer 1

Just create a function that takes names as arguments and constructs the expression for you. And then eval it each time by passing the names of each data.table you require. Here's an illustration:

get_expr <- function(x) {
    # 'x' is the names vector
    expr = paste0("i.", x)
    expr = lapply(expr, as.name)
    setattr(expr, 'names', x)
    as.call(c(quote(`:=`), expr))
}

> get_expr('value')    ## generates the required expression
# `:=`(value = i.value)

template[x, eval(get_expr("value"))]
template[y, eval(get_expr("value"))]

#     id1 id2       value
#  1:   a   1          NA
#  2:   a   2  0.01649728
#  3:   a   3 -0.27918482
#  4:   a   4 -1.16343900
#  5:   a   5          NA
#  6:   b   1          NA
#  7:   b   2          NA
#  8:   b   3  0.86933718
#  9:   b   4  2.26787200
# 10:   b   5  1.08325800