Make down SQL in rename column operations use the new name of the column (#354)

Ensure that 'alter column' operations that rename a column and also
specify `down` SQL (such as those that alter some other column attribute
at the time of the rename) must use the new name of the column in the
`down` SQL.

Without this change, the `down` SQL would require the use of the old
column name.

Fixes #350

examples/35_alter_column_multiple.json

@@ -18,7 +18,7 @@
           "constraint": "length(name) > 3"
         },
         "up": "(SELECT CASE WHEN name IS NULL OR LENGTH(name) <= 3 THEN 'placeholder' ELSE name END)",
-        "down": "name"
+        "down": "event_name"
       }
     }
   ]

pkg/migrations/op_alter_column.go

@@ -5,6 +5,7 @@ package migrations
 import (
 	"context"
 	"fmt"
+	"maps"
 
 	"github.com/lib/pq"
 	"github.com/xataio/pgroll/pkg/db"
@@ -57,11 +58,21 @@ func (o *OpAlterColumn) Start(ctx context.Context, conn db.DB, stateSchema strin
 			Name: TemporaryName(o.Column),
 		})
 
+		// If the column has been renamed, temporarily update the column name in
+		// the internal schema representation to ensure that the variable name in
+		// the down trigger corresponds to the new name of column.
+		cols := table.Columns
+		if o.Name != nil {
+			cols = maps.Clone(table.Columns)
+			cols[*o.Name] = cols[o.Column]
+			delete(cols, o.Column)
+		}
+
 		// Add a trigger to copy values from the new column to the old.
 		err = createTrigger(ctx, conn, tr, triggerConfig{
 			Name:           TriggerName(o.Table, TemporaryName(o.Column)),
 			Direction:      TriggerDirectionDown,
-			Columns:        table.Columns,
+			Columns:        cols,
 			SchemaName:     s.Name,
 			TableName:      o.Table,
 			PhysicalColumn: o.Column,

pkg/migrations/op_alter_column_test.go

@@ -46,7 +46,7 @@ func TestAlterColumnMultipleSubOperations(t *testing.T) {
 							Table:    "events",
 							Column:   "name",
 							Up:       "(SELECT CASE WHEN name IS NULL OR LENGTH(name) <= 3 THEN 'placeholder' ELSE name END)",
-							Down:     "name",
+							Down:     "event_name",
 							Name:     ptr("event_name"),
 							Type:     ptr("text"),
 							Comment:  nullable.NewNullableWithValue("the name of the event"),
@@ -319,7 +319,7 @@ func TestAlterColumnMultipleSubOperations(t *testing.T) {
 							Table:  "events",
 							Column: "name",
 							Up:     "name || '-' || random()*999::int",
-							Down:   "name",
+							Down:   "event_name",
 							Name:   ptr("event_name"),
 							Unique: &migrations.UniqueConstraint{
 								Name: "events_event_name_unique",