Why am I getting the error "db_name must be a string or symbol"?

Question

Quick version (for those familiar with Mongoid & Sinatra): If it's not the Psyche/Syck YAML-parsing issue, why else might I get this error when trying to connect to a MongoDB database using Mongoid? (Or maybe it is that issue, in which case, how do I fix my mongoid.yml file, posted below?)

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More detailed (original) version:

I have a Sinatra app interacting with a MongoDB database via Mongoid:

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configure do
    Mongoid.load!('config/mongoid.yml')
end

And my mongoid.yml file looks like this:

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development:
    host: localhost
    database: project_development

test:
    host: localhost
    database: project_test

production:
    uri: <%= ENV['MONGOLAB_URI'] %>

Whenever I try to interact with the database in some way, I get the error db_name must be a string or symbol.

Now, I have found plenty of information on Google about this; but everything I can find seems to indicate that the problem has to do with Ruby now using the Psyche YAML parser instead of the old Syck parser. I don't think that's actually relevant in my case because, as far as I can tell, the above YAML should be perfectly parsable by either.

(For what it's worth, though, I have tried using the YAML::ENGINE.yamler= 'syck' trick, to no avail. I got the exact same error message.)

When I change the configuration to this:

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Mongoid.configure do |config|
    name = "project_development"
    host = "localhost"
    config.master = Mongo::Connection.new.db(name)
end

...then everything works fine. So I know that MongoDB is working on my machine. It's specifically when I use a YAML file that things go awry.

So what gives?

Answer 1

Make sure that ENV['RACK_ENV'] is set properly, as that is what Mongoid.load! uses if it doesn't find Rails.env.