feat(models): add support for generated and identity columns

* extend Column struct with Generated, GenerationExpression, Identity, and IdentityGeneration fields
* update queryColumns to handle new column properties
* modify migration writer to avoid altering generated and identity columns
* add tests to ensure correct handling of generated and identity columns

pkg/readers/pgsql/queries.go

@@ -243,7 +243,11 @@ func (r *Reader) queryColumns(schemaName string) (map[string]map[string]*models.
 			c.numeric_scale,
 			c.udt_name,
 			pg_catalog.format_type(a.atttypid, a.atttypmod) as formatted_data_type,
-			col_description((c.table_schema||'.'||c.table_name)::regclass, c.ordinal_position) as description
+			col_description((c.table_schema||'.'||c.table_name)::regclass, c.ordinal_position) as description,
+			c.is_generated,
+			c.generation_expression,
+			c.is_identity,
+			c.identity_generation
 		FROM information_schema.columns c
 		JOIN pg_catalog.pg_namespace n
 			ON n.nspname = c.table_schema
@@ -268,17 +272,36 @@ func (r *Reader) queryColumns(schemaName string) (map[string]map[string]*models.
 	columnsMap := make(map[string]map[string]*models.Column)
 
 	for rows.Next() {
-		var schema, tableName, columnName, isNullable, dataType, udtName, formattedDataType string
+		var schema, tableName, columnName, isNullable, dataType, udtName, formattedDataType, isGenerated, isIdentity string
 		var ordinalPosition int
-		var columnDefault, description *string
+		var columnDefault, description, generationExpression, identityGeneration *string
 		var charMaxLength, numPrecision, numScale *int
 
-		if err := rows.Scan(&schema, &tableName, &columnName, &ordinalPosition, &columnDefault, &isNullable, &dataType, &charMaxLength, &numPrecision, &numScale, &udtName, &formattedDataType, &description); err != nil {
+		if err := rows.Scan(&schema, &tableName, &columnName, &ordinalPosition, &columnDefault, &isNullable, &dataType, &charMaxLength, &numPrecision, &numScale, &udtName, &formattedDataType, &description, &isGenerated, &generationExpression, &isIdentity, &identityGeneration); err != nil {
 			return nil, err
 		}
 
 		column := models.InitColumn(columnName, tableName, schema)
 
+		// GENERATED ALWAYS ... STORED columns are computed by Postgres and cannot
+		// have their default altered/dropped like a regular column.
+		column.Generated = isGenerated == "ALWAYS"
+		if generationExpression != nil {
+			column.GenerationExpression = *generationExpression
+		}
+
+		// GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY columns are driven by an
+		// internal sequence rather than a literal default (unlike serial columns,
+		// they carry no pg_attrdef row at all), so they need the same DB-side
+		// handling as generated columns even though the underlying mechanism differs.
+		column.Identity = isIdentity == "YES"
+		if identityGeneration != nil {
+			column.IdentityGeneration = strings.ToUpper(strings.TrimSpace(*identityGeneration))
+		}
+		if column.Identity {
+			column.AutoIncrement = true
+		}
+
 		// Check if this is a serial type (has nextval default)
 		hasNextval := false
 		if columnDefault != nil {
@@ -301,6 +324,24 @@ func (r *Reader) queryColumns(schemaName string) (map[string]map[string]*models.
 			column.Description = *description
 		}
 
+		if column.Generated {
+			note := "GENERATED ALWAYS AS (" + column.GenerationExpression + ") STORED"
+			if column.Description != "" {
+				column.Description = column.Description + " " + note
+			} else {
+				column.Description = note
+			}
+		}
+
+		if column.Identity {
+			note := "GENERATED " + column.IdentityGeneration + " AS IDENTITY"
+			if column.Description != "" {
+				column.Description = column.Description + " " + note
+			} else {
+				column.Description = note
+			}
+		}
+
 		if charMaxLength != nil {
 			column.Length = *charMaxLength
 		}