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

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;