Anything in the &default section can be merged in using <<: *default
In this case, the default already specifies that the adapter is
postgresql, the timeout is 5000, the encoding is unicode. It also
already specified the db pool, but the default actually checked 2
different ENVs and provided a fallback if it's not provided.
Lastly, I was able to move the prepared_statements section to the
defaults, because the .env.vagrant file doesn't specify
ENV['PREPARED_STATEMENTS'], development still just sets it to
'false'.
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