title: R flatten nested data.table
body: <p>I have nested data table, what is the best way to flatten nested tables to one data.table? Note, that nested tables can have unequal length, so tidyr::unnest() does not work.</p>
  
  <p>Code example:</p>
  
  <pre><code>NestedTable &lt;- data.table(
    COLUMN1 = c('var1', 'var2','var3'),
    COLUMN2 = c('col2a', 'col2b', 'col2c')
  )
  
  # add nested data.tables
  NestedTable[ , NESTED_COL := list(list(data.table(
    COLUMN4 = c(
      'A', 'B'
    ), 
    COLUMN5 = c(
      'C', 'D'
    )
  )))]
  
  NestedTable[ , NESTED_COL2 := list(list(data.table(
    COLUMN6 = c(
      'A', 'B','C'
    ), 
    COLUMN7 = c(
      'C', 'D','E'
    )
  )))]
  
  #      COLUMN1 COLUMN2 COLUMN4 COLUMN5 COLUMN6 COLUMN7
  # 1:    var1   col2a   A       C       A       C
  # 2:    var1   col2a   B       D       B       D
  # 3:    var1   col2a                   C       E
  # 4:    var2   col2b   A       C       A       C
  # 5:    var2   col2b   B       D       B       D
  # 6:    var2   col2b                   C       E
  # 7:    var3   col2c   A       C       A       C
  # 8:    var3   col2c   B       D       B       D
  # 9:    var3   col2c                   C       E
  </code></pre>