fix: skip FK constraint drop when referenced table is dropped with CASCADE (#382)

When dropping a table with CASCADE, PostgreSQL automatically removes FK
constraints on other tables that reference it. The diff generator was also
emitting explicit ALTER TABLE DROP CONSTRAINT for these same FKs, causing
"constraint does not exist" errors during apply.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tianzhou

internal/diff/diff.go

@@ -1644,7 +1644,7 @@ func (d *ddlDiff) generateModifySQL(targetSchema string, collector *diffCollecto
 	generateModifySequencesSQL(d.modifiedSequences, targetSchema, collector)
 
 	// Modify tables
-	generateModifyTablesSQL(d.modifiedTables, targetSchema, collector)
+	generateModifyTablesSQL(d.modifiedTables, d.droppedTables, targetSchema, collector)
 
 	// Find views that depend on views being recreated (issue #268, #308)
 	// Handles both materialized views and regular views with RequiresRecreate

internal/diff/table.go

@@ -536,11 +536,17 @@ func generateDeferredConstraintsSQL(deferred []*deferredConstraint, targetSchema
 }
 
 // generateModifyTablesSQL generates ALTER TABLE statements
-func generateModifyTablesSQL(diffs []*tableDiff, targetSchema string, collector *diffCollector) {
+func generateModifyTablesSQL(diffs []*tableDiff, droppedTables []*ir.Table, targetSchema string, collector *diffCollector) {
+	// Build a set of tables being dropped (CASCADE will remove their dependent FK constraints)
+	droppedTableSet := make(map[string]bool, len(droppedTables))
+	for _, t := range droppedTables {
+		droppedTableSet[t.Schema+"."+t.Name] = true
+	}
+
 	// Diffs are already sorted by the Diff operation
 	for _, diff := range diffs {
 		// Pass collector to generateAlterTableStatements to collect with proper context
-		diff.generateAlterTableStatements(targetSchema, collector)
+		diff.generateAlterTableStatements(targetSchema, collector, droppedTableSet)
 	}
 }
 
@@ -654,9 +660,20 @@ func shouldDeferConstraint(table *ir.Table, constraint *ir.Constraint, currentKe
 // generateAlterTableStatements generates SQL statements for table modifications
 // Note: DroppedTriggers are skipped here because they are already processed in the DROP phase
 // (see generateDropTriggersFromModifiedTables in trigger.go)
-func (td *tableDiff) generateAlterTableStatements(targetSchema string, collector *diffCollector) {
+// droppedTableSet contains "schema.table" keys for tables being dropped with CASCADE;
+// FK constraints referencing these tables are skipped since CASCADE already removes them.
+func (td *tableDiff) generateAlterTableStatements(targetSchema string, collector *diffCollector, droppedTableSet map[string]bool) {
 	// Drop constraints first (before dropping columns) - already sorted by the Diff operation
 	for _, constraint := range td.DroppedConstraints {
+		// Skip FK constraints whose referenced table is being dropped with CASCADE,
+		// since the CASCADE will already remove the constraint. (#382)
+		if constraint.Type == ir.ConstraintTypeForeignKey && constraint.ReferencedTable != "" {
+			refKey := constraint.ReferencedSchema + "." + constraint.ReferencedTable
+			if droppedTableSet[refKey] {
+				continue
+			}
+		}
+
 		tableName := getTableNameWithSchema(td.Table.Schema, td.Table.Name, targetSchema)
 		sql := fmt.Sprintf("ALTER TABLE %s DROP CONSTRAINT %s;", tableName, ir.QuoteIdentifier(constraint.Name))
 

testdata/diff/create_table/issue_382_drop_table_cascade_constraint/diff.sql

@@ -0,0 +1,3 @@
+DROP TABLE IF EXISTS b CASCADE;
+
+ALTER TABLE a DROP COLUMN b_id;

testdata/diff/create_table/issue_382_drop_table_cascade_constraint/new.sql

@@ -0,0 +1 @@
+CREATE TABLE a (id bigint PRIMARY KEY);

testdata/diff/create_table/issue_382_drop_table_cascade_constraint/old.sql

@@ -0,0 +1,5 @@
+CREATE TABLE a (id bigint PRIMARY KEY, b_id bigint);
+
+CREATE TABLE b (id bigint PRIMARY KEY);
+
+ALTER TABLE a ADD CONSTRAINT "a_b_id_fkey" FOREIGN KEY ("b_id") REFERENCES "b" ("id");

testdata/diff/create_table/issue_382_drop_table_cascade_constraint/plan.json

@@ -0,0 +1,26 @@
+{
+  "version": "1.0.0",
+  "pgschema_version": "1.8.0",
+  "created_at": "1970-01-01T00:00:00Z",
+  "source_fingerprint": {
+    "hash": "62dd4ce9d3f9c6521a16cdbf81cf9511ce4c5a18c65272f5dfa306bfe476cad3"
+  },
+  "groups": [
+    {
+      "steps": [
+        {
+          "sql": "DROP TABLE IF EXISTS b CASCADE;",
+          "type": "table",
+          "operation": "drop",
+          "path": "public.b"
+        },
+        {
+          "sql": "ALTER TABLE a DROP COLUMN b_id;",
+          "type": "table.column",
+          "operation": "drop",
+          "path": "public.a.b_id"
+        }
+      ]
+    }
+  ]
+}

testdata/diff/create_table/issue_382_drop_table_cascade_constraint/plan.sql

@@ -0,0 +1,3 @@
+DROP TABLE IF EXISTS b CASCADE;
+
+ALTER TABLE a DROP COLUMN b_id;

testdata/diff/create_table/issue_382_drop_table_cascade_constraint/plan.txt

@@ -0,0 +1,16 @@
+Plan: 1 to modify, 1 to drop.
+
+Summary by type:
+  tables: 1 to modify, 1 to drop
+
+Tables:
+  ~ a
+    - b_id (column)
+  - b
+
+DDL to be executed:
+--------------------------------------------------
+
+DROP TABLE IF EXISTS b CASCADE;
+
+ALTER TABLE a DROP COLUMN b_id;