[Fix #200] Make SchemaLoader aware of Rails 5 expression index

[pocke]

This pull request fixes #200

The error occurred on Rails 5 expression index.

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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[koic]

Thanks!