How to Exporting the data from postgres

This example uses a single table, but a more complex result is possible as well, where several tables are joined. Here is an example script:

COPY (
	SELECT
	userid AS "userId.auto()",
	'some-static-string' AS "__classname__.auto()",
	failed_login_count AS "failedLoginCount.int32()",
	CASE WHEN mail_validated=TRUE THEN 'TRUE' ELSE 'FALSE' END AS "mailValidated.boolean()",
	-42 AS "answers.toTheQuestion.int32()",
	'Slate Star Codex' AS "answers.niceBlog.auto()"
	FROM
	users.accounts
	) TO '/tmp/users-accounts.csv' WITH (FORMAT CSV, HEADER TRUE, DELIMITER E'\t');

And now for the import, which — thanks to our preparations — is a breeze:

mongoimport -d db -c users --type tsv --file export.tsv --headerline --columnsHaveTypes

Conclusion

Migrating data from postgresql — or any other sql server — is easy enough and very performant with native commandline tools. If you do need to have more control over the data format, you obviously will run into problems soon enough, where a migration script based on python or nodejs might be better suited than a big one-liner.