feat(pgsql): unique constraints on apply, matching unique index names, FK deferral

- migration writer now emits unique constraints (direct apply and live diff
  previously dropped them); idempotent via the existing DO block
- unique constraint adopts a same-named model unique index with
  ADD CONSTRAINT ... UNIQUE USING INDEX instead of building a second index
- dbml reader names unnamed unique indexes ukey_<table>_<cols> to match the
  unique constraint naming
- foreign key deferral: Constraint.Deferrable/InitiallyDeferred are *bool
  (nil = unspecified, never diffed); writers emit DEFERRABLE, DEFERRABLE
  INITIALLY DEFERRED and NOT DEFERRABLE; live reader reads condeferrable and
  condeferred; dbml reader/writer support deferrable and initially deferred
- tests and README/naming docs updated

pkg/models/deferral.go

@@ -0,0 +1,48 @@
+package models
+
+// DeferralClause returns the SQL deferral clause for a foreign key constraint
+// ("DEFERRABLE", "DEFERRABLE INITIALLY DEFERRED" or "NOT DEFERRABLE"), or ""
+// when the model does not specify one. InitiallyDeferred without Deferrable
+// implies DEFERRABLE; InitiallyDeferred=false alone specifies nothing.
+func (d *Constraint) DeferralClause() string {
+	deferrable, initially, set := d.effectiveDeferral()
+	switch {
+	case !set:
+		return ""
+	case !deferrable:
+		return "NOT DEFERRABLE"
+	case initially != nil && *initially:
+		return "DEFERRABLE INITIALLY DEFERRED"
+	default:
+		return "DEFERRABLE"
+	}
+}
+
+// effectiveDeferral resolves the optional flags. set is false when the model says nothing
+// about deferral; initially is nil when the initial mode is unspecified.
+func (d *Constraint) effectiveDeferral() (deferrable bool, initially *bool, set bool) {
+	switch {
+	case d.Deferrable != nil:
+		return *d.Deferrable, d.InitiallyDeferred, true
+	case d.InitiallyDeferred != nil && *d.InitiallyDeferred:
+		return true, d.InitiallyDeferred, true
+	}
+	return false, nil, false
+}
+
+// DeferralMatches reports whether two constraints agree on deferral. An unspecified side
+// matches anything, and the initial mode is only compared when both sides specify it.
+func DeferralMatches(a, b *Constraint) bool {
+	da, ia, aSet := a.effectiveDeferral()
+	db, ib, bSet := b.effectiveDeferral()
+	if !aSet || !bSet {
+		return true
+	}
+	if da != db {
+		return false
+	}
+	if !da || ia == nil || ib == nil {
+		return true
+	}
+	return *ia == *ib
+}

pkg/models/deferral_test.go

@@ -0,0 +1,59 @@
+package models
+
+import "testing"
+
+func boolPtr(b bool) *bool { return &b }
+
+func TestConstraint_DeferralClause(t *testing.T) {
+	tests := []struct {
+		name string
+		c    Constraint
+		want string
+	}{
+		{"unspecified", Constraint{}, ""},
+		{"deferrable", Constraint{Deferrable: boolPtr(true)}, "DEFERRABLE"},
+		{"deferrable initially deferred", Constraint{Deferrable: boolPtr(true), InitiallyDeferred: boolPtr(true)}, "DEFERRABLE INITIALLY DEFERRED"},
+		{"deferrable initially immediate", Constraint{Deferrable: boolPtr(true), InitiallyDeferred: boolPtr(false)}, "DEFERRABLE"},
+		{"initially deferred implies deferrable", Constraint{InitiallyDeferred: boolPtr(true)}, "DEFERRABLE INITIALLY DEFERRED"},
+		{"initially immediate alone is unspecified", Constraint{InitiallyDeferred: boolPtr(false)}, ""},
+		{"explicitly not deferrable", Constraint{Deferrable: boolPtr(false)}, "NOT DEFERRABLE"},
+		{"not deferrable ignores initially deferred", Constraint{Deferrable: boolPtr(false), InitiallyDeferred: boolPtr(true)}, "NOT DEFERRABLE"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.c.DeferralClause(); got != tt.want {
+				t.Errorf("DeferralClause() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestDeferralMatches(t *testing.T) {
+	live := func(def, init bool) *Constraint {
+		return &Constraint{Deferrable: boolPtr(def), InitiallyDeferred: boolPtr(init)}
+	}
+	tests := []struct {
+		name         string
+		model, other *Constraint
+		want         bool
+	}{
+		{"model unspecified vs deferrable live", &Constraint{}, live(true, false), true},
+		{"model unspecified vs not deferrable live", &Constraint{}, live(false, false), true},
+		{"deferrable vs deferrable live", &Constraint{Deferrable: boolPtr(true)}, live(true, true), true},
+		{"deferrable vs not deferrable live", &Constraint{Deferrable: boolPtr(true)}, live(false, false), false},
+		{"not deferrable vs deferrable live", &Constraint{Deferrable: boolPtr(false)}, live(true, false), false},
+		{"initially deferred vs initially immediate live", &Constraint{InitiallyDeferred: boolPtr(true)}, live(true, false), false},
+		{"initially deferred vs initially deferred live", &Constraint{InitiallyDeferred: boolPtr(true)}, live(true, true), true},
+		{"explicit initially immediate vs initially deferred live", &Constraint{Deferrable: boolPtr(true), InitiallyDeferred: boolPtr(false)}, live(true, true), false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := DeferralMatches(tt.model, tt.other); got != tt.want {
+				t.Errorf("DeferralMatches() = %v, want %v", got, tt.want)
+			}
+			if got := DeferralMatches(tt.other, tt.model); got != tt.want {
+				t.Errorf("DeferralMatches() reversed = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}

pkg/models/models.go

@@ -329,8 +329,8 @@ type Constraint struct {
 	ReferencedColumns []string       `json:"referenced_columns" yaml:"referenced_columns" xml:"referenced_columns"`
 	OnDelete          string         `json:"on_delete" yaml:"on_delete" xml:"on_delete"` // CASCADE, SET NULL, RESTRICT, etc.
 	OnUpdate          string         `json:"on_update" yaml:"on_update" xml:"on_update"`
-	Deferrable        bool           `json:"deferrable,omitempty" yaml:"deferrable,omitempty" xml:"deferrable,omitempty"`
-	InitiallyDeferred bool           `json:"initially_deferred,omitempty" yaml:"initially_deferred,omitempty" xml:"initially_deferred,omitempty"`
+	Deferrable        *bool          `json:"deferrable,omitempty" yaml:"deferrable,omitempty" xml:"deferrable,omitempty"`                         // nil: unspecified, left as is
+	InitiallyDeferred *bool          `json:"initially_deferred,omitempty" yaml:"initially_deferred,omitempty" xml:"initially_deferred,omitempty"` // nil: unspecified, left as is
 	Sequence          uint           `json:"sequence,omitempty" yaml:"sequence,omitempty" xml:"sequence,omitempty"`
 	GUID              string         `json:"guid" yaml:"guid" xml:"guid"`
 }