Fix retrieve_indexes_from_table when indexes is empty and base table does not exist. (#849)

Some tables may have a table_name_prefix but no indexes. Previous versions of
the code would strip the prefix and look for indexes on the resulting table 
which likely would not exist. This causes DB errors, at least in MySQL. So now
check if the new table exists first before trying to show its indexes.

Author: jjowdy

lib/annotate/annotate_models.rb

@@ -124,7 +124,11 @@ def retrieve_indexes_from_table(klass)
 
       # Try to search the table without prefix
       table_name_without_prefix = table_name.to_s.sub(klass.table_name_prefix, '')
-      klass.connection.indexes(table_name_without_prefix)
+      if klass.connection.table_exists?(table_name_without_prefix)
+        klass.connection.indexes(table_name_without_prefix)
+      else
+        []
+      end
     end
 
     # Use the column information in an ActiveRecord class

spec/lib/annotate/annotate_models_spec.rb

@@ -53,7 +53,8 @@ def mock_connection(indexes = [], foreign_keys = [], check_constraints = [])
            foreign_keys: foreign_keys,
            check_constraints: check_constraints,
            supports_foreign_keys?: true,
-           supports_check_constraints?: true)
+           supports_check_constraints?: true,
+           table_exists?: true)
   end
 
   # rubocop:disable Metrics/ParameterLists
@@ -538,7 +539,7 @@ def mock_column(name, type, options = {})
                   end
                 end
 
-                context 'when one of indexes includes orderd index key' do
+                context 'when one of indexes includes ordered index key' do
                   let :columns do
                     [
                       mock_column("id", :integer),
@@ -694,6 +695,24 @@ def mock_column(name, type, options = {})
                   it 'returns schema info without index information' do
                     is_expected.to eq expected_result
                   end
+
+                  # rubocop:disable RSpec/NestedGroups
+                  context 'when the unprefixed table name does not exist' do
+                    let :klass do
+                      mock_class(:users, primary_key, columns, indexes, foreign_keys).tap do |mock_klass|
+                        allow(mock_klass).to receive(:table_name_prefix).and_return('my_prefix_')
+                        allow(mock_klass.connection).to receive(:table_exists?).with('users').and_return(false)
+                        allow(mock_klass.connection).to receive(:indexes).with('users').and_raise('error fetching indexes on nonexistent table')
+                      end
+                    end
+
+                    it 'returns schema info without index information' do
+                      is_expected.to eq expected_result
+                      expect(klass).to have_received(:table_name_prefix).at_least(:once)
+                      expect(klass.connection).to have_received(:table_exists?).with('users')
+                    end
+                  end
+                  # rubocop:enable RSpec/NestedGroups
                 end
               end