It's not direct answer rather telling our experiences on how we design databases with Rails.
In fact, we never design tables. We design our model. When we make the plan of the application , we plan about the models and their relationships. During the creation of model, we define the fields & their types and rails model generator creates migration file for that. when we run
rake db:migrate the database structure is laid. however, after creation of the structure, we use phpMyAdmin (for MySQL) and pgAdmin (for postgres) to see the structures from time to time during the application development. if we need to readjust the fields in future, we do not manually edit the tables rather we create a separate migration (rails migration generator) where we design the changes and
rake db:migrate again adjusts the changes.
The above are for the SQL solutions. For NoSQL, so far we've used MongoDB (mongoid). As MongoDB is schemaless, table (collection) definition is easier. In the model, we define the fields and their type.
So, we never felt like we need any other visualized database softwares. Life is already much fun here.