Improve have_db_index to better handle columns with multiple indexes (

#1542)

The `have_db_index` matcher currently matches on the first index
that has the matching column names. But when used with the `unique`
qualifier that can result in the "wrong" index being matched causing
the test to fail. For example, one unique BTree index, and another
non-unique partial or GIN index.

lib/shoulda/matchers/active_record/have_db_index_matcher.rb

@@ -197,17 +197,28 @@ def correct_unique?
         def matched_index
           @_matched_index ||=
             if expected_columns.one?
-              actual_indexes.detect do |index|
+              sorted_indexes.detect do |index|
                 Array.wrap(index.columns) == expected_columns
               end
             else
-              actual_indexes.detect do |index|
+              sorted_indexes.detect do |index|
                 index.columns == expected_columns
               end
             end
         end
 
-        def actual_indexes
+        def sorted_indexes
+          if qualifiers.include?(:unique)
+            # return indexes with unique matching the qualifier first
+            unsorted_indexes.sort_by do |index|
+              index.unique == qualifiers[:unique] ? 0 : 1
+            end
+          else
+            unsorted_indexes
+          end
+        end
+
+        def unsorted_indexes
           model.connection.indexes(table_name)
         end
 

spec/unit/shoulda/matchers/active_record/have_db_index_matcher_spec.rb

@@ -155,6 +155,36 @@
             unique: true,
             qualifier_args: [],
           )
+
+          context 'when there are multiple possible matching columns' do
+            let(:model) do
+              define_model(
+                'Employee',
+                { ssn: :string },
+                parent_class: DevelopmentRecord,
+                customize_table: -> (table) {
+                  table.index(:ssn, name: 'aaa', unique: false)
+                  table.index(:ssn, name: 'bbb', unique: true)
+                  table.index(:ssn, name: 'ccc', unique: false)
+                },
+              )
+            end
+
+            it 'matches in the positive' do
+              expect(model.new).to have_db_index(:ssn).unique
+            end
+
+            it 'does not match in the negative' do
+              assertion = lambda do
+                expect(model.new).not_to have_db_index(:ssn).unique
+              end
+
+              expect(&assertion).to fail_with_message(<<-MESSAGE)
+Expected the employees table not to have a unique index on :ssn, but it
+does.
+              MESSAGE
+            end
+          end
         end
 
         context 'when qualified with unique: true' do