SQLite ORM

SQLite ORM light header only library for modern C++

Advantages

• No raw string queries
• Intuitive syntax
• Comfortable interface - one code line per single query
• Built with modern C++14 features (no macros)
• CRUD support
• Pure select query support
• STL compatible
• Custom types binding support
• BLOB support - maps to std::vector<char> or one can bind your custom type
• FOREIGN KEY support
• Composite key support
• JOIN support
• Transactions support
• Migrations functionality
• Powerful conditions
• ORDER BY and LIMIT, OFFSET support
• GROUP BY / DISTINCT support
• INDEX support
• Follows single responsibility principle - no need write code inside your data model classes
• Easy integration - single header only lib.
• The only dependency - libsqlite3
• C++ standard code style
• No undefined behaviour - if something goes wrong lib throws an exception
• In memory database support - provide :memory: or empty filename

sqlite_orm library allows to create easy data model mappings to your database schema. It is built to manage (CRUD) objects with a single column with primary key and without it. It also allows you to specify table names and column names explicitly no matter how your classes actually named. Take a look at example:

struct User{
    int id;
    std::string firstName;
    std::string lastName;
    int birthDate;
    std::shared_ptr<std::string> imageUrl;      
    int typeId;
};

struct UserType {
    int id;
    std::string name;
};

So we have database with predefined schema like

CREATE TABLE users (id integer primary key autoincrement, first_name text not null, last_name text not null, birth_date integer not null, image_url text, type_id integer not null)

CREATE TABLE user_types (id integer primary key autoincrement, name text not null DEFAULT 'name_placeholder')

Now we tell sqlite_orm library about schema and provide database filename. We create storage service object that has CRUD interface. Also we create every table and every column. All code is intuitive and minimalistic.

using namespace sqlite_orm;
auto storage = make_storage("db.sqlite",
                            make_table("users",
                                       make_column("id",
                                                   &User::id,
                                                   autoincrement(),
                                                   primary_key()),
                                       make_column("first_name",
                                                   &User::firstName),
                                       make_column("last_name",
                                                   &User::lastName),
                                       make_column("birth_date",
                                                   &User::birthDate),
                                       make_column("image_url",
                                                   &User::imageUrl),
                                       make_column("type_id",
                                                   &User::typeId)),
                            make_table("user_types",
                                       make_column("id",
                                                   &UserType::id,
                                                   autoincrement(),
                                                   primary_key()),
                                       make_column("name",
                                                   &UserType::name,
                                                   default_value("name_placeholder"))));

Too easy isn't it? You do not have to specify mapped type explicitly - it is deduced from your member pointers you pass during making a column (for example: &User::id). To create a column you have to pass two arguments at least: its name in the table and your mapped class member pointer. You can also add extra arguments to tell your storage about column's constraints like not_null (deduced from type), primary_key, autoincrement, default_value or unique(order isn't important).

If your datamodel classes have private or protected members to map to sqlite then you can make a storage with setter and getter functions. More info in the example.

More details about making storage can be found in tutorial.

CRUD

Let's create and insert new User into database. First we need to create a User object with any id and call insert function. It will return id of just created user or throw exception if something goes wrong.

User user{-1, "Jonh", "Doe", 664416000, std::make_shared<std::string>("url_to_heaven"), 3 };
    
auto insertedId = storage.insert(user);
cout << "insertedId = " << insertedId << endl;      //  insertedId = 8
user.id = insertedId;

User secondUser{-1, "Alice", "Inwonder", 831168000, {} , 2};
insertedId = storage.insert(secondUser);
secondUser.id = insertedId;

Next let's get our user by id.

try{
    auto user = storage.get<User>(insertedId);
    cout << "user = " << user.firstName << " " << user.lastName << endl;
}catch(sqlite_orm::not_found_exception) {
    cout << "user not found with id " << insertedId << endl;
}catch(...){
    cout << "unknown exeption" << endl;
}

Probably you may not like throwing exceptions. Me too. Exception not_found_exception is thrown because return type in get function is not nullable. You can use alternative version get_no_throw which returns std::shared_ptr and doesn't throw not_found_exception if nothing found - just returns nullptr.

if(auto user = storage.get_no_throw<User>(insertedId)){
    cout << "user = " << user->firstName << " " << user->lastName << endl;
}else{
    cout << "no user with id " << insertedId << endl;
}

std::shared_ptr is used as optional in sqlite_orm. Of course there is class optional in C++14 located at std::experimental::optional. But we don't want to use it until it is experimental.

We can also update our user. It updates row by id provided in user object and sets all other non primary_key fields to values stored in the passed user object. So you can just assign members to user object you want and call update

user.firstName = "Nicholas";
user.imageUrl = "https://cdn1.iconfinder.com/data/icons/man-icon-set/100/man_icon-21-512.png"
storage.update(user);

Also there is a non-CRUD update version update_all:

storage.update_all(set(&User::lastName, "Hardey",
                       &User::typeId, 2),
                   where(c(&User::firstName) == "Tom"));

And delete. To delete you have to pass id only, not whole object. Also we need to explicitly tell which class of object we want to delete. Function name is remove not delete cause delete is a reserved word in C++.

storage.remove<User>(insertedId)

Also we can extract all objects into std::vector.

auto allUsers = storage.get_all<User>();
cout << "allUsers (" << allUsers.size() << "):" << endl;
for(auto &user : allUsers) {
    cout << storage.dump(user) << endl; //  dump returns std::string with json-like style object info. For example: { id : '1', first_name : 'Jonh', last_name : 'Doe', birth_date : '664416000', image_url : 'https://cdn1.iconfinder.com/data/icons/man-icon-set/100/man_icon-21-512.png', type_id : '3' }
}

And one can specify return container type explicitly: let's get all users in std::list, not std::vector:

auto allUsersList = storage.get_all<User, std::list<User>>();

Container must be STL compatible (must have push_back(T&&) function in this case).

get_all can be too heavy for memory so you can iterate row by row (i.e. object by object):

for(auto &user : storage.iterate<User>()) {
    cout << storage.dump(user) << endl;
}

iterate member function returns adapter object that has begin and end member functions returning iterators that fetch object on dereference operator call.

CRUD functions get, get_no_throw, remove, update (not insert) work only if your type has a primary key column. If you try to get an object that is mapped to your storage but has no primary key column a std::runtime_error will be thrown cause sqlite_orm cannot detect an id. If you want to know how to perform a storage without primary key take a look at key_value.cpp example in examples folder.

Aggregate Functions

//  SELECT AVG(id) FROM users
auto averageId = storage.avg(&User::id);    
cout << "averageId = " << averageId << endl;        //  averageId = 4.5
    
//  SELECT AVG(birth_date) FROM users
auto averageBirthDate = storage.avg(&User::birthDate);  
cout << "averageBirthDate = " << averageBirthDate << endl;      //  averageBirthDate = 6.64416e+08
  
//  SELECT COUNT(*) FROM users
auto usersCount = storage.count<User>();    
cout << "users count = " << usersCount << endl;     //  users count = 8

//  SELECT COUNT(id) FROM users
auto countId = storage.count(&User::id);    
cout << "countId = " << countId << endl;        //  countId = 8

//  SELECT COUNT(image_url) FROM users
auto countImageUrl = storage.count(&User::imageUrl);   
cout << "countImageUrl = " << countImageUrl << endl;      //  countImageUrl = 5

//  SELECT GROUP_CONCAT(id) FROM users
auto concatedUserId = storage.group_concat(&User::id);      
cout << "concatedUserId = " << concatedUserId << endl;      //  concatedUserId = 1,2,3,4,5,6,7,8

//  SELECT GROUP_CONCAT(id, "---") FROM users
auto concatedUserIdWithDashes = storage.group_concat(&User::id, "---");     
cout << "concatedUserIdWithDashes = " << concatedUserIdWithDashes << endl;      //  concatedUserIdWithDashes = 1---2---3---4---5---6---7---8

//  SELECT MAX(id) FROM users
if(auto maxId = storage.max(&User::id)){    
    cout << "maxId = " << *maxId <<endl;    //  maxId = 12  (maxId is std::shared_ptr<int>)
}else{
    cout << "maxId is null" << endl;
}
    
//  SELECT MAX(first_name) FROM users
if(auto maxFirstName = storage.max(&User::firstName)){ 
    cout << "maxFirstName = " << *maxFirstName << endl; //  maxFirstName = Jonh (maxFirstName is std::shared_ptr<std::string>)
}else{
    cout << "maxFirstName is null" << endl;
}

//  SELECT MIN(id) FROM users
if(auto minId = storage.min(&User::id)){    
    cout << "minId = " << *minId << endl;   //  minId = 1 (minId is std::shared_ptr<int>)
}else{
    cout << "minId is null" << endl;
}

//  SELECT MIN(last_name) FROM users
if(auto minLastName = storage.min(&User::lastName)){
    cout << "minLastName = " << *minLastName << endl;   //  minLastName = Doe
}else{
    cout << "minLastName is null" << endl;
}

//  SELECT SUM(id) FROM users
if(auto sumId = storage.sum(&User::id)){    //  sumId is std::shared_ptr<int>
    cout << "sumId = " << *sumId << endl;
}else{
    cout << "sumId is null" << endl;
}

//  SELECT TOTAL(id) FROM users
auto totalId = storage.total(&User::id);
cout << "totalId = " << totalId << endl;    //  totalId is double (always)

Where conditions

You also can select objects with custom where conditions with =, !=, >, >=, <, <=, IN, BETWEEN and LIKE.

For example: let's select users with id lesser than 10:

//  SELECT * FROM users WHERE id < 10
auto idLesserThan10 = storage.get_all<User>(where(c(&User::id) < 10));
cout << "idLesserThan10 count = " << idLesserThan10.size() << endl;
for(auto &user : idLesserThan10) {
    cout << storage.dump(user) << endl;
}

Or select all users who's first name is not equal "John":

//  SELECT * FROM users WHERE first_name != 'John'
auto notJohn = storage.get_all<User>(where(c(&User::firstName) != "John"));
cout << "notJohn count = " << notJohn.size() << endl;
for(auto &user : notJohn) {
    cout << storage.dump(user) << endl;
}

By the way one can implement not equal in a different way using C++ negation operator:

auto notJohn2 = storage.get_all<User>(where(not (c(&User::firstName) == "John")));

You can use ! and not in this case cause they are equal. Also you can chain several conditions with and and or operators. Let's try to get users with query with conditions like where id >= 5 and id <= 7 and not id = 6:

auto id5and7 = storage.get_all<User>(where(c(&User::id) <= 7 and c(&User::id) >= 5 and not (c(&User::id) == 6)));
cout << "id5and7 count = " << id5and7.size() << endl;
for(auto &user : id5and7) {
    cout << storage.dump(user) << endl;
}

Or let's just export two users with id 10 or id 16 (of course if these users exist):

auto id10or16 = storage.get_all<User>(where(c(&User::id) == 10 or c(&User::id) == 16));
cout << "id10or16 count = " << id10or16.size() << endl;
for(auto &user : id10or16) {
    cout << storage.dump(user) << endl;
}

In fact you can chain together any number of different conditions with any operator from and, or and not. All conditions are templated so there is no runtime overhead. And this makes sqlite_orm the most powerful sqlite C++ ORM library!

Moreover you can use parentheses to set the priority of query conditions:

auto cuteConditions = storage.get_all<User>(where((c(&User::firstName) == "John" or c(&User::firstName) == "Alex") and c(&User::id) == 4));  //  where (first_name = 'John' or first_name = 'Alex') and id = 4
cout << "cuteConditions count = " << cuteConditions.size() << endl; //  cuteConditions count = 1
cuteConditions = storage.get_all<User>(where(c(&User::firstName) == "John" or (c(&User::firstName) == "Alex" and c(&User::id) == 4)));   //  where first_name = 'John' or (first_name = 'Alex' and id = 4)
cout << "cuteConditions count = " << cuteConditions.size() << endl; //  cuteConditions count = 2

Also we can implement get by id with get_all and where like this:

//  SELECT * FROM users WHERE ( 2 = id )
auto idEquals2 = storage.get_all<User>(where(2 == c(&User::id)));
cout << "idEquals2 count = " << idEquals2.size() << endl;
if(idEquals2.size()){
    cout << storage.dump(idEquals2.front()) << endl;
}else{
    cout << "user with id 2 doesn't exist" << endl;
}

Lets try the IN operator:

//  SELECT * FROM users WHERE id IN (2, 4, 6, 8, 10)
auto evenLesserTen10 = storage.get_all<User>(where(in(&User::id, {2, 4, 6, 8, 10})));
cout << "evenLesserTen10 count = " << evenLesserTen10.size() << endl;
for(auto &user : evenLesserTen10) {
    cout << storage.dump(user) << endl;
}

//  SELECT * FROM users WHERE last_name IN ("Doe", "White")
auto doesAndWhites = storage.get_all<User>(where(in(&User::lastName, {"Doe", "White"})));
cout << "doesAndWhites count = " << doesAndWhites.size() << endl;
for(auto &user : doesAndWhites) {
    cout << storage.dump(user) << endl;
}

And BETWEEN:

//  SELECT * FROM users WHERE id BETWEEN 66 AND 68
auto betweenId = storage.get_all<User>(where(between(&User::id, 66, 68)));
cout << "betweenId = " << betweenId.size() << endl;
for(auto &user : betweenId) {
    cout << storage.dump(user) << endl;
}

And even LIKE:

//  SELECT * FROM users WHERE last_name LIKE 'D%'
auto whereNameLike = storage.get_all<User>(where(like(&User::lastName, "D%")));
cout << "whereNameLike = " << whereNameLike.size() << endl;
for(auto &user : whereNameLike) {
    cout << storage.dump(user) << endl;
}

Looks like magic but it works very simple. Cute function c (column) takes a class pointer and returns a special expression middle object that can be used with operators overloaded in ::sqlite_orm namespace. Operator overloads act just like functions

• is_equal
• is_not_equal
• greater_than
• greater_or_equal
• lesser_than
• lesser_or_equal
• is_null
• is_not_null

that simulate binary comparison operator so they take 2 arguments: left hand side and right hand side. Arguments may be either member pointer of mapped class or any other expression (core function or literal). Binary comparison functions map arguments to text to be passed to sqlite engine to process query. Member pointers are being mapped to column names and literals to literals (numbers to raw numbers and string to quoted strings). Next where function places brackets around condition and adds "WHERE" keyword before condition text. Next resulted string appends to query string and is being processed further.

If you omit where function in get_all it will return all objects from a table:

auto allUsers = storage.get_all<User>();

Also you can use remove_all function to perform DELETE FROM ... WHERE query with the same type of conditions.

storage.remove_all<User>(where(c(&User::id) < 100));

Raw select

If you need to extract only a single column (SELECT %column_name% FROM %table_name% WHERE %conditions%) you can use a non-CRUD select function:

//  SELECT id FROM users
auto allIds = storage.select(&User::id);    
cout << "allIds count = " << allIds.size() << endl; //  allIds is std::vector<int>
for(auto &id : allIds) {
    cout << id << " ";
}
cout << endl;

//  SELECT id FROM users WHERE last_name = 'Doe'
auto doeIds = storage.select(&User::id, where(c(&User::lastName) == "Doe"));
cout << "doeIds count = " << doeIds.size() << endl; //  doeIds is std::vector<int>
for(auto &doeId : doeIds) {
    cout << doeId << " ";
}
cout << endl;

//  SELECT last_name FROM users WHERE id < 300
auto allLastNames = storage.select(&User::lastName, where(c(&User::id) < 300));    
cout << "allLastNames count = " << allLastNames.size() << endl; //  allLastNames is std::vector<std::string>
for(auto &lastName : allLastNames) {
    cout << lastName << " ";
}
cout << endl;

//  SELECT id FROM users WHERE image_url IS NULL
auto idsWithoutUrls = storage.select(&User::id, where(is_null(&User::imageUrl)));
for(auto id : idsWithoutUrls) {
    cout << "id without image url " << id << endl;
}

//  SELECT id FROM users WHERE image_url IS NOT NULL
auto idsWithUrl = storage.select(&User::id, where(is_not_null(&User::imageUrl)));
for(auto id : idsWithUrl) {
    cout << "id with image url " << id << endl;
}
auto idsWithUrl2 = storage.select(&User::id, where(not is_null(&User::imageUrl)));
assert(std::equal(idsWithUrl2.begin(),
                  idsWithUrl2.end(),
                  idsWithUrl.begin()));

Also you're able to select several column in a vector of tuples. Example:

//  `SELECT first_name, last_name FROM users WHERE id > 250 ORDER BY id`
auto partialSelect = storage.select(columns(&User::firstName, &User::lastName),
                                    where(c(&User::id) > 250),
                                    order_by(&User::id));
cout << "partialSelect count = " << partialSelect.size() << endl;
for(auto &t : partialSelect) {
    auto &firstName = std::get<0>(t);
    auto &lastName = std::get<1>(t);
    cout << firstName << " " << lastName << endl;
}

ORDER BY support

ORDER BY query option can be applied to get_all and select functions just like where but with order_by function. It can be mixed with WHERE in a single query. Examples:

//  `SELECT * FROM users ORDER BY id`
auto orderedUsers = storage.get_all<User>(order_by(&User::id));
cout << "orderedUsers count = " << orderedUsers.size() << endl;
for(auto &user : orderedUsers) {
    cout << storage.dump(user) << endl;
}

//  `SELECT * FROM users WHERE id < 250 ORDER BY first_name`
auto orderedUsers2 = storage.get_all<User>(where(c(&User::id) < 250), order_by(&User::firstName));
cout << "orderedUsers2 count = " << orderedUsers2.size() << endl;
for(auto &user : orderedUsers2) {
    cout << storage.dump(user) << endl;
}

//  `SELECT * FROM users WHERE id > 100 ORDER BY first_name ASC`
auto orderedUsers3 = storage.get_all<User>(where(c(&User::id) > 100), order_by(&User::firstName).asc());
cout << "orderedUsers3 count = " << orderedUsers3.size() << endl;
for(auto &user : orderedUsers3) {
    cout << storage.dump(user) << endl;
}

//  `SELECT * FROM users ORDER BY id DESC`
auto orderedUsers4 = storage.get_all<User>(order_by(&User::id).desc());
cout << "orderedUsers4 count = " << orderedUsers4.size() << endl;
for(auto &user : orderedUsers4) {
    cout << storage.dump(user) << endl;
}

//  `SELECT first_name FROM users ORDER BY ID DESC`
auto orderedFirstNames = storage.select(&User::firstName, order_by(&User::id).desc());
cout << "orderedFirstNames count = " << orderedFirstNames.size() << endl;
for(auto &firstName : orderedFirstNames) {
    cout << "firstName = " << firstName << endl;
}

LIMIT and OFFSET

There are three available versions of LIMIT/OFFSET options:

• LIMIT %limit%
• LIMIT %limit% OFFSET %offset%
• LIMIT %offset%, %limit%

All these versions available with the same interface:

//  `SELECT * FROM users WHERE id > 250 ORDER BY id LIMIT 5`
auto limited5 = storage.get_all<User>(where(c(&User::id) > 250),
                                      order_by(&User::id),
                                      limit(5));
cout << "limited5 count = " << limited5.size() << endl;
for(auto &user : limited5) {
    cout << storage.dump(user) << endl;
}

//  `SELECT * FROM users WHERE id > 250 ORDER BY id LIMIT 5, 10`
auto limited5comma10 = storage.get_all<User>(where(c(&User::id) > 250),
                                             order_by(&User::id),
                                             limit(5, 10));
cout << "limited5comma10 count = " << limited5comma10.size() << endl;
for(auto &user : limited5comma10) {
    cout << storage.dump(user) << endl;
}

//  `SELECT * FROM users WHERE id > 250 ORDER BY id LIMIT 5 OFFSET 10`
auto limit5offset10 = storage.get_all<User>(where(c(&User::id) > 250),
                                            order_by(&User::id),
                                            limit(5, offset(10)));
cout << "limit5offset10 count = " << limit5offset10.size() << endl;
for(auto &user : limit5offset10) {
    cout << storage.dump(user) << endl;
}

Please beware that queries LIMIT 5, 10 and LIMIT 5 OFFSET 10 mean different. LIMIT 5, 10 means LIMIT 10 OFFSET 5.

JOIN support

You can perform simple JOIN, CROSS JOIN, INNER JOIN, LEFT JOIN or LEFT OUTER JOIN in your query. Instead of joined table specify mapped type. Example for doctors and visits:

//  SELECT a.doctor_id, a.doctor_name,
//      c.patient_name, c.vdate
//  FROM doctors a
//  LEFT JOIN visits c
//  ON a.doctor_id=c.doctor_id;
auto rows = storage2.select(columns(&Doctor::id, &Doctor::name, &Visit::patientName, &Visit::vdate),
                            left_join<Visit>(on(c(&Doctor::id) == &Visit::doctorId)));  //  one `c` call is enough cause operator overloads are templated
for(auto &row : rows) {
    cout << std::get<0>(row) << '\t' << std::get<1>(row) << '\t' << std::get<2>(row) << '\t' << std::get<3>(row) << endl;
}
cout << endl;

Simple JOIN:

//  SELECT a.doctor_id,a.doctor_name,
//      c.patient_name,c.vdate
//  FROM doctors a
//  JOIN visits c
//  ON a.doctor_id=c.doctor_id;
rows = storage2.select(columns(&Doctor::id, &Doctor::name, &Visit::patientName, &Visit::vdate),
                       join<Visit>(on(c(&Doctor::id) == &Visit::doctorId)));
for(auto &row : rows) {
    cout << std::get<0>(row) << '\t' << std::get<1>(row) << '\t' << std::get<2>(row) << '\t' << std::get<3>(row) << endl;
}
cout << endl;

Two INNER JOINs in one query:

//  SELECT
//      trackid,
//      tracks.name AS Track,
//      albums.title AS Album,
//      artists.name AS Artist
//  FROM
//      tracks
//  INNER JOIN albums ON albums.albumid = tracks.albumid
//  INNER JOIN artists ON artists.artistid = albums.artistid;
auto innerJoinRows2 = storage.select(columns(&Track::trackId, &Track::name, &Album::title, &Artist::name),
                                     inner_join<Album>(on(c(&Album::albumId) == &Track::albumId)),
                                     inner_join<Artist>(on(c(&Artist::artistId) == &Album::artistId)));
//  innerJoinRows2 is std::vector<std::tuple<decltype(&Track::trackId), decltype(&Track::name), decltype(&Album::title), decltype(&Artist::name)>>

More join examples can be found in examples folder.

Migrations functionality

There are no explicit up and down functions that are used to be used in migrations. Instead sqlite_orm offers sync_schema function that takes responsibility of comparing actual db file schema with one you specified in make_storage call and if something is not equal it alters or drops/creates schema.

storage.sync_schema();
//  or
storage.sync_schema(true);

Please beware that sync_schema doesn't guarantee that data will be saved. It tries to save it only. Below you can see rules list that sync_schema follows during call:

• if there are excess tables exist in db they are ignored (not dropped)
• every table from storage is compared with it's db analog and
  • if table doesn't exist it is created
  • if table exists its colums are being compared with table_info from db and
    • if there are columns in db that do not exist in storage (excess) table will be dropped and recreated if preserve is false, and table will be copied into temporary table without excess columns, source table will be dropped, copied table will be renamed to source table (sqlite remove column technique) if preserve is true. preserve is the first argument in sync_schema function. It's default value is false. Beware that setting it to true may take time for copying table rows.
    • if there are columns in storage that do not exist in db they will be added using 'ALTER TABLE ... ADD COLUMN ...' command and table data will not be dropped but if any of added columns is null but has not default value table will be dropped and recreated
    • if there is any column existing in both db and storage but differs by any of properties (type, pk, notnull) table will be dropped and recreated (dflt_value isn't checked cause there can be ambiguity in default values, please beware).

The best practice is to call this function right after storage creation.

Transactions

There are three ways to begin and commit/rollback transactions:

• explicitly call begin_transaction();, rollback(); or commit(); functions
• use transaction function which begins transaction implicitly and takes a lambda argument which returns true for commit and false for rollback. All storage calls performed in lambda can be commited or rollbacked by returning true or false.
• use transaction_guard function which returns a guard object which works just like lock_guard for std::mutex.

Example for explicit call:

auto secondUser = storage.get<User>(2);

storage.begin_transaction();
secondUser.typeId = 3;
storage.update(secondUser);
storage.rollback(); //  or storage.commit();

secondUser = storage.get<decltype(secondUser)>(secondUser.id);
assert(secondUser.typeId != 3);

Example for implicit call:

storage.transaction([&] () mutable {    //  mutable keyword allows make non-const function calls
    auto secondUser = storage.get<User>(2);
    secondUser.typeId = 1;
    storage.update(secondUser);
    auto gottaRollback = bool(rand() % 2);
    if(gottaRollback){  //  dummy condition for test
        return false;   //  exits lambda and calls ROLLBACK
    }
    return true;        //  exits lambda and calls COMMIT
});

The second way guarantees that commit or rollback will be called. You can use either way.

Trancations are useful with changes sqlite function that returns number of rows modified.

storage.transaction([&] () mutable {
    storage.remove_all<User>(where(c(&User::id) < 100));
    auto usersRemoved = storage.changes();
    cout << "usersRemoved = " << usersRemoved << endl;
    return true;
});

It will print a number of deleted users (rows). But if you call changes without a transaction and your database is located in file not in RAM the result will be 0 always cause sqlite_orm opens and closes connection every time you call a function without a transaction.

Also a transaction function returns true if transaction is commited and false if it is rollbacked. It can be useful if your next moves depend on transaction result:

auto commited = storage.transaction([&] () mutable {    
    auto secondUser = storage.get<User>(2);
    secondUser.typeId = 1;
    storage.update(secondUser);
    auto gottaRollback = bool(rand() % 2);
    if(gottaRollback){  //  dummy condition for test
        return false;   //  exits lambda and calls ROLLBACK
    }
    return true;        //  exits lambda and calls COMMIT
});
if(commited){
    cout << "Commited successfully, go on." << endl;
}else{
    cerr << "Commit failed, process an error" << endl;
}

Example for transaction_guard function:

try{
  auto guard = storage.transaction_guard(); //  calls BEGIN TRANSACTION and returns guard object
  user.name = "Paul";
  auto notExisting = storage.get<User>(-1); //  exception is thrown here, guard calls ROLLBACK in its destructor
  guard.commit();
}catch(...){
  cerr << "exception" << endl;
}

In memory database

To manage in memory database just provide :memory: or "" instead as filename to make_storage.

Notes

To work well your data model class must be default constructable and must not have const fields mapped to database cause they are assigned during queries. Otherwise code won't compile on line with member assignment operator.

For more details please check the project wiki.

Installation

Just put include/sqlite_orm/sqlite_orm.h into you folder with headers. Also it is recommended to keep project libraries' sources in separate folders cause there is no normal dependency manager for C++ yet.

Requirements

• C++14 compatible compiler (not C++11 cause of templated lambdas in the lib).
• libsqlite3 linked to your binary