原文链接： https://www.cnblogs.com/Evsward/p/multi_index.html
智能合约操作链数据库是很常见的应用场景。EOS提供了专门的工具来做这件事（相当于Ethereum的leveldb），专业术语叫做持久化API，本文将完整严密地介绍这个工具以及对它的使用测试。
关键字：EOS，智能合约，链数据库，eosio::multi_index，constructor，emplace，erase，find。
需求
首先来看EOS中智能合约涉及到持久化的场景需求。一个action在执行时会有上下文变量出现，包括事务机制的处理，这些内容会应用链上分配的内存资源，而如果没有持久化技术，执行超过作用域时就会丢失掉这些上下文数据。因此要使用持久化技术将关键内容记录在链数据库中，任何时候使用都不受影响。持久化技术应该包括：
- 记录一些状态持久化到数据库中
- 具备查询的能力从数据库中获取内容
- 提供C++ 的API来调用这些服务，也服务于合约开发者
eosio::multi_index
这是模仿boost::multi_index开发的一套库。它使用C++编写，提供了合约与数据库的交互持久化接口。
Multi-Index Iterators：不同于其他key-value数据库，multi_index提供了不同类型的key对应的值也是可迭代的复杂集合类型。创建表
- class/struct 定义一个持久化对象。
- 该对象需要有一个const的成员作为主键，类型为uint64_t
- 二级主键可选，提供不同的键类型
- 为每个二级索引定义一个键导出器，键导出器是一个函数，可以用来从Multi_index表中获取键
使用Multi-Index表
一般来讲，对数据库的操作无外乎增删改查，
- 增加对应的方法是emplace
- 修改就是modify
- 删除是erase
- 查找包括find和get，以及迭代器操作
实战
下面我们通过一个智能合约操作底层数据库的实例，来演示对持久化api，multi_index的使用。过程中，也会展示相应api的定义。
车辆维修跟踪
首先，创建一个service表，用来创建服务记录报告，它包含的字段有：
- 主键，客户ID不可当做主键，因为一个客户可以有多条记录。实际上，我们并不直接需要主键，所以我们可以让系统为我们生成一个。
- 客户ID，与账户名字对应
- 服务日期
- 里程表，汽车里程表
#include
using namespace eosio;
class vehicle : public eosio::contract {
public:
/// @abi table
struct service_rec {
uint64_t pkey;
account_name customer;
uint32_t service_date;
uint32_t odometer;
auto primary_key() const { return pkey; }
account_name get_customer() const { return customer; }
EOSLIB_SERIALIZE(service_rec, (pkey)(customer)(service_date)(odometer))
};
typedef multi_index service_table_type;
using contract::contract;
/// @abi action
void exec(account_name owner, account_name customer) {
print("Hello, ", name{customer});
service_table_type service_table(current_receiver(), owner);// 构造器，第一个参数是code代表表的拥有者，目前是current_receiver()，也可以使用contract基类的构造器初始化时的账户名_self，第二个参数是scope，在代码层次范围的标识符，这里就使用传入的owner账户。
uint64_t pkeyf;// 主键
service_table.emplace(owner, [&](auto &s_rec) {
s_rec.pkey = service_table.available_primary_key();// 主键自增
pkeyf = s_rec.pkey;
print(pkeyf);// 打印主键内容
s_rec.customer = customer;
s_rec.service_date = 2000;
s_rec.odometer = 1000;
});
service_rec result = service_table.get(pkeyf);
print("_", result.pkey);
print("_", result.customer);
print("_", result.service_date);
print("_", result.odometer);
}
};
EOSIO_ABI(vehicle, (exec)) 使用eosiocpp工具生成abi文件和wast文件。
eosiocpp -g vehicle.abi vehicle.cpp | eosiocpp -o vehicle.wasm vehicle.cpp
然后在终端中部署该合约，
cleos set contract one work/CLionProjects/github.com/eos/contracts/vehicle
调用合约的exec方法，并输出结果：
liuwenbin@liuwenbin-H81M-DS2:~$ cleos push action one exec '["one","two"]' -p one
executed transaction: 3a45eaeb06732ad0c53ba7b157003e1c503f74ed447029d82cecbe12926cc9a9 112 bytes 365 us
# one <= one::exec {"owner":"one","customer":"two"}
>> Hello, two13_13_14927180964919508992_2000_1000
warning: transaction executed locally, but may not be confirmed by the network yet通过输出结果，可以知道主键为13，customer账户名被转为无符号32位整型数据14927180964919508992，服务时间为2000，里程表为1000。我们已经成功将数据存入了multi_index并取了出来。
删除的话可以通过service_table.erase(result);来删除掉对应记录。
find涉及二级索引，迭代器等操作，end判断等multi_index的api操作没有给出具体实例，未来在其他合约使用时会直接说明。再演练一个例子
为了更好的熟悉multi_index的机制，我们再演练一个简单的例子：维护一个todolist的数据库表。直接上代码如下：
#include
#include
class todolist : public eosio::contract
{
public:
using eosio::contract::contract;
// @abi table todos i64
struct todo
{
uint64_t id; // 待办事项主键id
std::string description; // 待办事项的描述参数
uint64_t completed; // 标记一个待办事项是否已完成
uint64_t primary_key() const { return id; }
EOSLIB_SERIALIZE(todo, (id)(description)(completed))
};
typedef eosio::multi_index todo_table;
// @abi action
void create(account_name author, const uint32_t id, const std::string &description)
{
todo_table todos(_self, author);
todos.emplace(author, [&](auto &new_todo) {
new_todo.id = id;
new_todo.description = description;
new_todo.completed = 0;
});
eosio::print("todo#", id, " created");
}
// @abi action
void complete(account_name author, const uint32_t id)
{
todo_table todos(_self, author);
auto todo_lookup = todos.find(id);
eosio_assert(todo_lookup != todos.end(), "Todo does not exist");
todos.modify(todo_lookup, author, [&](auto &modifiable_todo) {
modifiable_todo.completed = 1;
});
eosio::print("todo#", id, " marked as complete");
}
// @abi action
void destroy(account_name author, const uint32_t id)
{
todo_table todos(_self, author);
auto todo_lookup = todos.find(id);
todos.erase(todo_lookup);
eosio::print("todo#", id, " destroyed");
}
};
EOSIO_ABI(todolist, (create)(complete)(destroy)) 这里加入了对数据的增删改查功能。然后我们使用
eosiocpp -o todolist.wast todolist.cpp && eosiocpp -g todolist.abi todolist.cpp
创建对应的abi和wast文件。接下来创建用户todo.user，
liuwenbin@liuwenbin-H81M-DS2:~$ cleos create account eosio todo.user EOS6MRyAjQq8ud7hVNYcfnVPJqcVpscN5So8BhtHuGYqET5GDW5CV EOS6MRyAjQq8ud7hVNYcfnVPJqcVpscN5So8BhtHuGYqET5GDW5CV
executed transaction: 7d7191e2935c6fd0024f571e228ee11d51f4f44a64ed2ce977326fb27679f0c6 200 bytes 174 us
# eosio <= eosio::newaccount {"creator":"eosio","name":"todo.user","owner":{"threshold":1,"keys":[{"key":"EOS6MRyAjQq8ud7hVNYcfnV...
warning: transaction executed locally, but may not be confirmed by the network yet部署合约，
liuwenbin@liuwenbin-H81M-DS2:~$ cleos set contract todo.user work/VSCode-Projects/eos/contracts/todolist -p todo.user
Reading WAST/WASM from work/VSCode-Projects/eos/contracts/todolist/todolist.wasm...
Using already assembled WASM...
Publishing contract...
executed transaction: 03befa58d6a54970db708beaa0520179277b01addf1ec647a76a9b3f6459ff57 5128 bytes 2203 us
# eosio <= eosio::setcode {"account":"todo.user","vmtype":0,"vmversion":0,"code":"0061736d01000000016e1260047f7e7f7f0060037f7e...
# eosio <= eosio::setabi {"account":"todo.user","abi":{"types":[],"structs":[{"name":"todo","base":"","fields":[{"name":"id",...
warning: transaction executed locally, but may not be confirmed by the network yet创建新条目，
liuwenbin@liuwenbin-H81M-DS2:~$ cleos push action todo.user create '["todo.user",1,"hello, world"]' -p todo.user
executed transaction: 54d9825971370a242fa51fa7cc587a6478dd7852039c263d91058c6b1163a4bc 120 bytes 336 us
# todo.user <= todo.user::create {"author":"todo.user","id":1,"description":"hello, world"}
>> todo#1 created查询，
liuwenbin@liuwenbin-H81M-DS2:~$ cleos get table todo.user todo.user todos
{
"rows": [{
"id": 1,
"description": "hello, world",
"completed": 0
}
],
"more": false
}完成事项，
liuwenbin@liuwenbin-H81M-DS2:~$ cleos push action todo.user complete '["todo.user",1]' -p todo.user
executed transaction: 11423637cb321969961b3ce0305c93ba7f95a83b6d82c1a4e31a08c569f2dcaa 104 bytes 312 us
# todo.user <= todo.user::complete {"author":"todo.user","id":1}
>> todo#1 marked as complete
warning: transaction executed locally, but may not be confirmed by the network yet再次查询，completed字段已置为1，
liuwenbin@liuwenbin-H81M-DS2:~$ cleos get table todo.user todo.user todos
{
"rows": [{
"id": 1,
"description": "hello, world",
"completed": 1
}
],
"more": false
}创建多条记录，
liuwenbin@liuwenbin-H81M-DS2:~$ cleos push action todo.user create '["todo.user",2,"eos funk"]' -p todo.user
executed transaction: 35f417a4d7438e6ea9ffd837e3c261fca0cb3926b534dc6603fdb38f94d5cd77 120 bytes 329 us
# todo.user <= todo.user::create {"author":"todo.user","id":2,"description":"eos funk"}
>> todo#2 created
warning: transaction executed locally, but may not be confirmed by the network yet
liuwenbin@liuwenbin-H81M-DS2:~$ cleos push action todo.user create '["todo.user",10,"go to bank"]' -p todo.user
executed transaction: cd4cd2c85500e93a79eb5c3b3852a0a96a9fb220696c63f2683b473d58a6ca34 120 bytes 311 us
# todo.user <= todo.user::create {"author":"todo.user","id":10,"description":"go to bank"}
>> todo#10 created
warning: transaction executed locally, but may not be confirmed by the network yet查询，
liuwenbin@liuwenbin-H81M-DS2:~$ cleos get table todo.user todo.user todos
{
"rows": [{
"id": 1,
"description": "hello, world",
"completed": 1
},{
"id": 2,
"description": "eos funk",
"completed": 0
},{
"id": 10,
"description": "go to bank",
"completed": 0
}
],
"more": false
}删除一条数据，
liuwenbin@liuwenbin-H81M-DS2:~$ cleos push action todo.user destroy '["todo.user",2]' -p todo.user
executed transaction: c87e37f4521e0ce2a865acbeb63c0f3a0681b9b5a0a6c5db8cd53584d6d13dca 104 bytes 2198 us
# todo.user <= todo.user::destroy {"author":"todo.user","id":2}
>> todo#2 destroyed
warning: transaction executed locally, but may not be confirmed by the network yet再次查询，
liuwenbin@liuwenbin-H81M-DS2:~$ cleos get table todo.user todo.user todos
{
"rows": [{
"id": 1,
"description": "hello, world",
"completed": 1
},{
"id": 10,
"description": "go to bank",
"completed": 0
}
],
"more": false
}这是一个完整的，通过multi_index进行curd的一个例子（第一个例子是官方文档给出的，其中内容有bug）。
总结
通过本篇文章的学习，我们掌握了如何在EOS中使用智能合约调用multi_index实现数据的持久化。
参考文档
- EOS官方文档
- EOS asia