I received the following error after upgrading my application to Rails 5 and it is somewhat cryptic:
...connection_specification.rb:170:in `spec': database configuration does not specify adapter (ActiveRecord::AdapterNotSpecified)
I found the solution to this problem, it turns out that in my case since I was connecting to multiple databases that there was a subtle change in what Rails 5 expected over Rails 4.
If you are connecting to multiple databases the establish_connection used within the model connecting to the separate database requires a symbol instead of a string in Rails 5.
Works
establish_connection :secondary_database
Where as the following no longer works:
establish_connection "secondary_database"
In my case some of my old database connections had used the string argument and were failing, causing me to think that there was an strange incompatibility between Rails 5 and my code base. I thought I would share this as I do not see it documented anywhere specifically.