let's warn with heredocs

The current style for warning messages without newlines uses
concatenation of string literals with manual trailing spaces
where needed.

Heredocs have better readability, and with `squish` we can still
produce a single line.

This is a similar use case to the one that motivated defining
`strip_heredoc`, heredocs are super clean.

1 files changed, 7 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/connection_specification.rb b/activerecord/lib/active_record/connection_adapters/connection_specification.rb
index e02824b33d..af405fc690 100644
--- a/activerecord/lib/active_record/connection_adapters/connection_specification.rb
+++ b/activerecord/lib/active_record/connection_adapters/connection_specification.rb
@@ -1,4 +1,5 @@
 require 'uri'
+require 'active_support/core_ext/string/filters'
 
 module ActiveRecord
   module ConnectionAdapters
@@ -221,8 +222,12 @@ module ActiveRecord
           # this ambiguous behaviour and in the future this function
           # can be removed in favor of resolve_url_connection.
           if configurations.key?(spec) || spec !~ /:/
-            ActiveSupport::Deprecation.warn "Passing a string to ActiveRecord::Base.establish_connection " \
-              "for a configuration lookup is deprecated, please pass a symbol (#{spec.to_sym.inspect}) instead"
+            ActiveSupport::Deprecation.warn(<<-MSG.squish)
+              Passing a string to ActiveRecord::Base.establish_connection for a
+              configuration lookup is deprecated, please pass a symbol
+              (#{spec.to_sym.inspect}) instead
+            MSG
+
             resolve_symbol_connection(spec)
           else
             resolve_url_connection(spec)