Adding array selector array[0]

[houshuang]

I have not written Ruby for a while, but I needed this for a project, and it seems to work. This adds the possibility of selecting a specific value using an array selector [0]. This is especially useful for projects where there are arrays where you always just want the first value, rather than creating a Postgres array.

[houshuang]

Note that currently this only works if the array selector is the final element. Ideally something like supervisor[0].id should also work.

[laurentrivard]

@houshuang Looking into converting our MongoDB data with postgres but we have arrays inside our documents. Would you PR help us import arrays into their own tables?

[houshuang]

No, this only selects a specific value from an array, it does not import the whole array.

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