fix: preserve MySQL schema fidelity

pkg/models/models.go

@@ -256,9 +256,19 @@ func (d *Column) SQLName() string {
 	return strings.ToLower(d.Name)
 }
 
+// IndexPart is an ordered MySQL index key part. Name and Expression are mutually
+// exclusive; expressions are executable SQL and are not identifier-normalized.
+type IndexPart struct {
+	Name       string `json:"name,omitempty" yaml:"name,omitempty" xml:"name,omitempty"`
+	Expression string `json:"expression,omitempty" yaml:"expression,omitempty" xml:"expression,omitempty"`
+	Length     int    `json:"length,omitempty" yaml:"length,omitempty" xml:"length,omitempty"`
+	Descending bool   `json:"descending,omitempty" yaml:"descending,omitempty" xml:"descending,omitempty"`
+}
+
 // Index represents a database index for optimizing query performance.
 // Indexes can be unique, partial, or include additional columns.
 type Index struct {
+	KeyParts    []IndexPart    `json:"key_parts,omitempty" yaml:"key_parts,omitempty" xml:"key_parts,omitempty"`
 	Name        string         `json:"name" yaml:"name" xml:"name"`
 	Description string         `json:"description,omitempty" yaml:"description,omitempty" xml:"description,omitempty"`
 	Table       string         `json:"table,omitempty" yaml:"table,omitempty" xml:"table,omitempty"`
@@ -318,6 +328,7 @@ func (d *Relationship) SQLName() string {
 // Constraint represents a database constraint that enforces data integrity rules.
 // Constraints can be primary keys, foreign keys, unique constraints, check constraints, or not-null constraints.
 type Constraint struct {
+	Metadata          map[string]any `json:"metadata,omitempty" yaml:"metadata,omitempty" xml:"-"`
 	Name              string         `json:"name" yaml:"name" xml:"name"`
 	Type              ConstraintType `json:"type" yaml:"type" xml:"type"`
 	Columns           []string       `json:"columns" yaml:"columns" xml:"columns"`