st: tricky data merge/joinby problem

["Dimitriy V. Masterov" <dvmaster@gmail.com>]

I have two files that I would like to merge. The first contains data
on city blocks and block groups (BGs) and fraction of population
variable. A simplified version of the data looks like this:

bid bgid  fracpop
11  1    .5
12  1    .5
21  2    .3
22  2    .2
23  2    .5

For example, BG 1 contains 2 blocks, each of which has half of the BG
1's population (fracpop==.5). The unique identifier in this file is
bid.

I would like to merge the data above with panel data file2 that
contains block group populations over time. This data looks like:

bgid dateyq bgpop
1    2010q1 100
1    2010q2 105
1    2010q3 106
1    2010q4 125

Here bgid and dateyq are the identifiers. The final goal of merging is
to come up with a population for each block by allocating bgpop using
the weights in fracpop. For example, for BG 1, this would yield:

bid bgid dateyq bpop
11  1    2010q1 50
12  1    2010q1 50

Does this require the dreaded m:m merge with bgid as the id as the
first step? That appears to work (although I only checked a few
cases). Or is is better to expand the first file into a panel and then
merge on bgid and dateyq? Or should I use -joinby bgid using
file2.dta-? I am not sure which is the most efficient solution.

DVM
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