Fix error message when adapter is not specified

When we added support for multiple databases through a 3-tiered config
and configuration objects this error message got a bit convoluted.

Previously if you had an application with a missing configuation and
multiple databases the error message would look like this:

```
'doesnexist' database is not configured. Available: development,
development, test, test, production, production
(ActiveRecord::AdapterNotSpecified)
```

That's not very descriptive since it duplicates the environments
(because there are multiple databases per environment for this
application).

To fix this I've constructed a bit more readable error message which now
reads like this if you have a multi db app:

```
The `doesntexist` database is not configured for the `production`
environment. (ActiveRecord::AdapterNotSpecified)

Available databases configurations are:

development: primary, primary_readonly
test: primary, primary_readonly
production: primary, primary_readonly
```

And like this if you have a single db app:

```
The `doesntexist` database is not configured for the `production`
environment. (ActiveRecord::AdapterNotSpecified)

Available databases configurations are:

development
test
```

This makes the error message more readable and presents the user all
available options for the database connections.

1 files changed, 20 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/connection_specification.rb b/activerecord/lib/active_record/connection_adapters/connection_specification.rb
index f60d8469cc..9eaf9d9a89 100644
--- a/activerecord/lib/active_record/connection_adapters/connection_specification.rb
+++ b/activerecord/lib/active_record/connection_adapters/connection_specification.rb
@@ -248,10 +248,29 @@ module ActiveRecord
             if db_config
               resolve_connection(db_config.config).merge("name" => pool_name.to_s)
             else
-              raise(AdapterNotSpecified, "'#{env_name}' database is not configured. Available: #{configurations.configurations.map(&:env_name).join(", ")}")
+              raise AdapterNotSpecified, <<~MSG
+                The `#{env_name}` database is not configured for the `#{ActiveRecord::ConnectionHandling::DEFAULT_ENV.call}` environment.
+
+                Available databases configurations are:
+
+                #{build_configuration_sentence}
+              MSG
             end
           end
 
+          def build_configuration_sentence # :nodoc:
+            configs = configurations.configs_for(include_replicas: true)
+
+            configs.group_by(&:env_name).map do |env, config|
+              namespaces = config.map(&:spec_name)
+              if namespaces.size > 1
+                "#{env}: #{namespaces.join(", ")}"
+              else
+                env
+              end
+            end.join("\n")
+          end
+
           # Accepts a hash. Expands the "url" key that contains a
           # URL database connection to a full connection
           # hash and merges with the rest of the hash.