Fix index_exists? for PostgreSQL expression indexes

Previously Rails expected indexes to be an array of columns, but for
PostgreSQL a expression index can just be a string of text. Handle this
by forcing `Index#columns` to be an Array inside `index_exists?`.

Closes #36739

1 files changed, 4 insertions, 2 deletions

diff --git a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
index d99593817a..830c0892d3 100644
--- a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
@@ -253,9 +253,11 @@ module ActiveRecord
 
       def test_expression_index
         with_example_table do
-          @connection.add_index "ex", "mod(id, 10), abs(number)", name: "expression"
+          expr = "mod(id, 10), abs(number)"
+          @connection.add_index "ex", expr, name: "expression"
           index = @connection.indexes("ex").find { |idx| idx.name == "expression" }
-          assert_equal "mod(id, 10), abs(number)", index.columns
+          assert_equal expr, index.columns
+          assert_equal true, @connection.index_exists?("ex", expr, name: "expression")
         end
       end