Add UUID helper function

UUID creates string literal expression from uuid object.
uuid can be any uuid type with a String method.

internal/jet/literal_expression.go

@@ -409,3 +409,9 @@ func Raw(raw string, parent ...Expression) Expression {
 
 	return rawExp
 }
+
+// UUID is a helper function to create string literal expression from uuid object
+// value can be any uuid type with a String method
+func UUID(value fmt.Stringer) StringExpression {
+	return String(value.String())
+}

mysql/literal.go

@@ -51,6 +51,10 @@ var Decimal = jet.Decimal
 // String creates new string literal expression
 var String = jet.String
 
+// UUID is a helper function to create string literal expression from uuid object
+// value can be any uuid type with a String method
+var UUID = jet.UUID
+
 // Date creates new date literal
 var Date = func(year int, month time.Month, day int) DateExpression {
 	return CAST(jet.Date(year, month, day)).AS_DATE()

postgres/literal.go

@@ -44,6 +44,10 @@ var Decimal = jet.Decimal
 // String creates new string literal expression
 var String = jet.String
 
+// UUID is a helper function to create string literal expression from uuid object
+// value can be any uuid type with a String method
+var UUID = jet.UUID
+
 // Bytea craates new bytea literal expression
 var Bytea = func(value string) StringExpression {
 	return CAST(jet.String(value)).AS_BYTEA()

tests/mysql/alltypes_test.go

@@ -78,7 +78,7 @@ func TestUUID(t *testing.T) {
 	require.NoError(t, err)
 	require.True(t, dest.StrUUID != nil)
 	require.True(t, dest.UUID.String() != uuid.UUID{}.String())
-	require.True(t, dest.StrUUID.String() != uuid.UUID{}.String())
+	require.Equal(t, dest.StrUUID.String(), "dc8daae3-b83b-11e9-8eb4-98ded00c39c6")
 	require.Equal(t, dest.StrUUID.String(), dest.BinUUID.String())
 	requireLogged(t, query)
 }

tests/postgres/sample_test.go

@@ -15,9 +15,12 @@ import (
 )
 
 func TestUUIDType(t *testing.T) {
+
+	id := uuid.MustParse("a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11")
+
 	query := AllTypes.
 		SELECT(AllTypes.UUID, AllTypes.UUIDPtr).
-		WHERE(AllTypes.UUID.EQ(String("a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11")))
+		WHERE(AllTypes.UUID.EQ(UUID(id)))
 
 	testutils.AssertDebugStatementSql(t, query, `
 SELECT all_types.uuid AS "all_types.uuid",