Add ability to change the names of the default handlers

When I wrote the `connected_to` and `connects_to` API's I wrote them
with the idea in mind that it didn't really matter what the
handlers/roles were called as long as those connecting to the roles knew
which one wrote and which one read.

With the introduction of the middleware Rails begins to assume it's
`writing` and `reading` and there's no room for other roles. At GitHub
we've been using this method for a long time so we have a ton of legacy
code that uses different handler names `default` and `readonly`. We
could rename all our code but I think this is better for a few reasons:

- Legacy apps that have been using multiple databases for a long time
  can have an eaiser time switching.
- If we later find this to cause more issues than it's worth we can
  easily deprecate.
- We won't force old apps to rewrite the resolver middleware just to use
  a different handler.

Adding the writing_role/reading_role required that I move the code that
creates the first handler for writing to the railtie. If I didn't move
this the core class would assign the handler before I was able to assign
a new one in my configuration and I'd end up with 3 handlers instead of
2.

1 files changed, 3 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/middleware/database_selector/resolver.rb b/activerecord/lib/active_record/middleware/database_selector/resolver.rb
index acdb9b3238..fc71835cea 100644
--- a/activerecord/lib/active_record/middleware/database_selector/resolver.rb
+++ b/activerecord/lib/active_record/middleware/database_selector/resolver.rb
@@ -45,7 +45,7 @@ module ActiveRecord
 
           def read_from_primary(&blk)
             ActiveRecord::Base.connection.while_preventing_writes do
-              ActiveRecord::Base.connected_to(role: :writing) do
+              ActiveRecord::Base.connected_to(role: ActiveRecord::Base.writing_role) do
                 instrumenter.instrument("database_selector.active_record.read_from_primary") do
                   yield
                 end
@@ -54,7 +54,7 @@ module ActiveRecord
           end
 
           def read_from_replica(&blk)
-            ActiveRecord::Base.connected_to(role: :reading) do
+            ActiveRecord::Base.connected_to(role: ActiveRecord::Base.reading_role) do
               instrumenter.instrument("database_selector.active_record.read_from_replica") do
                 yield
               end
@@ -62,7 +62,7 @@ module ActiveRecord
           end
 
           def write_to_primary(&blk)
-            ActiveRecord::Base.connected_to(role: :writing) do
+            ActiveRecord::Base.connected_to(role: ActiveRecord::Base.writing_role) do
               instrumenter.instrument("database_selector.active_record.wrote_to_primary") do
                 yield
               ensure