thisversion 0.1.0

Keep persisted schema history explicit while your application uses a single current model.
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145

use thisversion::traits::TryIntoModel;
use thisversion::{VersionBoundary, VersionFamily};

#[derive(Debug, PartialEq, Eq)]
enum MigrationError {
    EmptyName,
    ZeroTimeout,
}

#[derive(VersionFamily)]
#[thisversion(fallible)]
enum SettingsVersions {
    V1(SettingsV1),
    V2(SettingsV2),
    V3(SettingsV3),
}

struct SettingsV1 {
    name: String,
    timeout_secs: u32,
}

struct SettingsV2 {
    name: String,
    timeout_secs: u32,
    enabled: bool,
}

#[derive(VersionBoundary)]
#[thisversion(model = Settings, error = MigrationError)]
struct SettingsV3 {
    name: String,
    timeout_secs: u32,
    enabled: bool,
    retries: u8,
}

impl TryFrom<SettingsV1> for SettingsV2 {
    type Error = MigrationError;

    fn try_from(old: SettingsV1) -> Result<Self, Self::Error> {
        if old.name.trim().is_empty() {
            return Err(MigrationError::EmptyName);
        }

        Ok(Self {
            name: old.name,
            timeout_secs: old.timeout_secs,
            enabled: true,
        })
    }
}

impl TryFrom<SettingsV2> for SettingsV3 {
    type Error = MigrationError;

    fn try_from(old: SettingsV2) -> Result<Self, Self::Error> {
        if old.timeout_secs == 0 {
            return Err(MigrationError::ZeroTimeout);
        }

        Ok(Self {
            name: old.name,
            timeout_secs: old.timeout_secs,
            enabled: old.enabled,
            retries: 3,
        })
    }
}

#[derive(Debug, PartialEq, Eq)]
struct Settings {
    name: String,
    timeout_secs: u32,
    enabled: bool,
    retries: u8,
}

#[test]
fn oldest_version_runs_both_fallible_migrations() {
    let stored = SettingsVersions::V1(SettingsV1 {
        name: "worker".into(),
        timeout_secs: 30,
    });

    assert_eq!(
        stored.try_into_model(),
        Ok(Settings {
            name: "worker".into(),
            timeout_secs: 30,
            enabled: true,
            retries: 3,
        })
    );
}

#[test]
fn first_migration_reports_its_error() {
    let stored = SettingsVersions::V1(SettingsV1 {
        name: "  ".into(),
        timeout_secs: 30,
    });

    assert_eq!(stored.try_into_model(), Err(MigrationError::EmptyName));
}

#[test]
fn second_migration_reports_its_error_from_either_earlier_version() {
    let oldest = SettingsVersions::V1(SettingsV1 {
        name: "worker".into(),
        timeout_secs: 0,
    });

    // Check V1 passes through the second migration after the first succeeds:
    assert_eq!(oldest.try_into_model(), Err(MigrationError::ZeroTimeout));

    let previous = SettingsVersions::V2(SettingsV2 {
        name: "worker".into(),
        timeout_secs: 0,
        enabled: false,
    });

    // Check V2 enters the same failing transition directly:
    assert_eq!(previous.try_into_model(), Err(MigrationError::ZeroTimeout));
}

#[test]
fn latest_version_skips_historical_migrations() {
    let stored = SettingsVersions::V3(SettingsV3 {
        name: String::new(),
        timeout_secs: 0,
        enabled: false,
        retries: 1,
    });

    assert_eq!(
        stored.try_into_model(),
        Ok(Settings {
            name: String::new(),
            timeout_secs: 0,
            enabled: false,
            retries: 1,
        })
    );
}