This is kind of hard to explain but if you have a database config with another level like this: ``` development: primary: database: "my db" variables: statement_timeout: 1000 ``` the database configurations code would chooke on the `variables` level because it didn't know what to do with it. We'd see the following error: ``` lib/active_record/database_configurations.rb:72:in `block in find_db_config': undefined method `env_name' for [nil]:Array (NoMethodError) ``` The problem here is that Rails does correctly identify this as not a real configuration but returns `[nil]` along with the others. We need to make sure to flatten the array and remove all the `nil`'s before returning the `configurations` objects. Fixes #35646