前几天翻译了一篇关于EOS智能合约数据库的内容，今天来演示一下数据库的使用方法。 目录 增 查 改 删 新增 新增内容往往用到emplace构造函数，来进行数据库对象的新增。录

增
查
改
删
新增

新增内容往往用到emplace构造函数，来进行数据库对象的新增。

.cpp

void test_da::create(account_name user, string title, string content)

{
    require_auth( user ); //验证权限
    das datable( _self, user);　//定义数据库对象
    datable.emplace(user, [&]( da & d){
        d.title = title;
        d.content = content;
        d.post_id = datable.available_primary_key();
        d.poster = user;
    });　//数据库内容创建
}

这里需要注意的是：

定义数据库对象，　其中第一个参数是合约的拥有者_self，第二个变量就是数据库的payer，也就是数据库是谁的，数据库存储在谁的账户下。
emplace函数接收两个参数，一个payer,和一个lamada表达式。这个结构是固定的。
那么我们现在来看一下我们的test_da类是怎么定义的。

.hpp　

class test_da : public contract {

 public:
       test_da( account_name self ):contract(self){}

       // @abi action
       void create(account_name user, string title, string content);

 private:
       // @abi table data i64
       struct da {
             uint64_t     post_id;
             account_name poster;
             string       title;
             string       content;

             uint64_t primary_key()const { return post_id; }
             account_name get_poster() const { return poster; }

             EOSLIB_SERIALIZE(da, (post_id)(poster)(title)(content))
       };
       typedef eosio::multi_index<N(data), da, indexed_by<N(byposter), const_mem_fun<da, account_name, &da::get_poster>> > das;

};
} /// namespace eosio
所有的智能合约都继承自contract合约。`test_da( account_name self ):contract(self){}
是test_da`合约的构造函数。
下面是对create函数的声明。
接下来是对数据字段的定义。这里我们定义了数据结构da.
primary_key函数是定义主键的函数。　
接下来我们定义了辅助主键返回poster。
EOSLIB_SERIALIZE宏的第一个参数是数据结构，其他参数是数据结构中的数据成员。
typedef我们在这里定义了一个名字为das的类型，它用来定义数据库对象。这里我们定义的是一个具有主键及一个辅助键的数据库对象。
.abi　
abi 非常重要，错误的abi会导致合约执行失败。

{
"types": [],
"structs": [{

  "name": "da",
  "base": "",
  "fields": [{
      "name": "post_id",
      "type": "uint64"
    },{
      "name": "poster",
      "type": "account_name"
    },{
      "name": "title",
      "type": "string"
    },{
      "name": "content",
      "type": "string"
    }
  ]
},{
  "name": "create",
  "base": "",
  "fields": [{
      "name": "user",
      "type": "account_name"
    },{
      "name": "title",
      "type": "string"
    },{
      "name": "content",
      "type": "string"
    }
  ]
}}

],
"actions": [{

      "name": "create",
      "type": "create",
    }

],
"tables": [{

  "name": "data",
  "index_type": "i64",
  "key_names": [
    "post_id"
  ],
  "key_types": [
    "uint64"
  ],
  "type": "da"
}

],
"ricardian_clauses": []
}
接下来来演示一下:

发布合约
cleos set contract eosio test_da
Reading WAST/WASM from test_da/test_da.wasm...
Using already assembled WASM...
Publishing contract...
executed transaction: 3d6f04278617d3807fe876a33057f1155acf9c9e5a392ac6ed8ad51e79506009 6752 bytes 24679 us

eosio <= eosio::setcode {"account":"eosio","vmtype":0,"vmversion":0,"code":"0061736d0100000001ad011a60037f7e7e0060057f7e7e7f...

eosio <= eosio::setabi {"account":"eosio","abi":{"types":[],"structs":[{"name":"da","base":"","fields":[{"name":"post_id","...

创建数据
cleos push action eosio create '{"user":"eosio","title":"first","content":"create a first one"}' -p eosio
executed transaction: 830057f270fa499b1d61b82e80ad8cda1774cdc1786c1e786f558a3e0a48974c 216 bytes 17229 us

eosio <= eosio::create {"user":"eosio","title":"first","content":"create a first one"}

下面我们来查一下数据表：

cleos get table eosio eosio data
{
"rows": [{

  "post_id": 0,
  "poster": "eosio",
  "title": "first",
  "content": "create a first one"
}

],
"more": false
}
创建信息成功，那么我们用别的账号创建会怎样呢？

cleos push action eosio create '{"user":"eostea","title":"eostea first","content":"eostea create a first one"}' -p eostea
executed transaction: 8542a87e563a9c62b7dbe46ae09ccf829c7821f8879167066b658096718de148 232 bytes 2243 us

eosio <= eosio::create {"user":"eostea","title":"eostea first","content":"eostea create a first one"}

查看数据表：

cleos get table eosio eostea data
{
"rows": [{

  "post_id": 0,
  "poster": "eostea",
  "title": "eostea first",
  "content": "eostea create a first one"
}

],
"more": false
}
到这里，相信大家对创建数据已经没有什么疑惑了。

查询

对于数据库，最重要的功能就是查询，如果没有查询功能，数据库里的数据就不能呈现，也就没有意义。查询数据库主要分为两方面，一方面是主键查询，一方面是通过二级索引查询。
这里为了使表中数据多样化我会做一些修改，将所有数据都合到一张表中。
我将以上.cpp中的das datable( _self, user);改为das datable( _self, _self);.这样数据都存在合约账户的表中。

主键查询

这里我添加了一个方法来查询数据并打印：

void test_da::getd(uint64_t post_id){

    das datable(_self, _self);
    auto post_da = datable.find( post_id);
    eosio::print("Post_id: ", post_da->post_id, "  Post_Tile: ", post_da->title.c_str(), " Content: ", post_da->content.c_str());
}

abi文件也做了相应的调整：

执行：

cleos push action eosio getd '{"post_id":1}' -p eosio
executed transaction: ac8663235462d947c74542af848cca54a059c3991d193237025da7d4767d6725 192 bytes 1724 us

eosio <= eosio::getd {"post_id":1}

Post_id: 1 Post_Tile: first Content: eosio create a first one
二级索引查询

添加二级索引查询代码如下：

auto poster_index = datable.template get_index();
auto pos = poster_index.find( user );

for (; pos != poster_index.end(); pos++)
{

eosio::print("content:", pos->content.c_str(), " post_id:", pos->post_id, " title:", pos->title.c_str());

}
获取二级索引并获取数据，这里我只用了find查询，其他查询就不在一一介绍。

执行getd操作：

cleos push action eosio getd '{"post_id":2,"user": "eostea"}' -p eosio
executed transaction: 2370e1fb1ee8a581f7321f02fb40645e51269e579d183c33ef470dba0b3afdbc 200 bytes 5403 us

eosio <= eosio::getd {"post_id":2,"user":"eostea"}

Post_id: 2 Post_Tile: eostea first Content: eostea create a first onecontent:eostea create a first one post_id:2 title:eostea first
数据库中数据如下：

cleos get table eosio eosio data
{
"rows": [{

 "post_id": 0,
 "poster": "eosio",
 "title": "first",
 "content": "eostea create a first one"

},{

 "post_id": 1,
 "poster": "eosio",
 "title": "first",
 "content": "eostea create a first one"

},{

 "post_id": 2,
 "poster": "eostea",
 "title": "eostea first",
 "content": "eostea create a first one"

}
],
"more": false
}
更改

更改数据库内容。
这里我们先看一下目前的数据库内容。

cleos get table eosio eosio data
{
"rows": [{

 "post_id": 0,
 "poster": "eosio",
 "title": "first",
 "content": "eostea create a first one"

},{

 "post_id": 1,
 "poster": "eosio",
 "title": "first",
 "content": "eostea create a first one"

},{

 "post_id": 2,
 "poster": "eostea",
 "title": "eostea first",
 "content": "eostea create a first one"

}
],
"more": false
}
写一个更改数据的action代码如下：

void test_da::change(account_name user, uint64_t post_id, string title, string content)

{
    require_auth(user);
    das datable( _self, user);
    auto post = datable.find(post_id);
    eosio_assert(post->poster == user, "yonghucuowu");
    datable.modify(post, user, [&](auto& p){
        if (title != "")
            p.title = title;
        if (content != "")
            p.content = content;
    });
}

1.　前几行代码已经之前讲解过，现在直接说modify方法，他的第一个参数是你查询出的要更改的对象，第二个参数是payer,其他的不用多说。
下面我们执行一下命令：

cleos push action eosio change '{"user":"eosio","post_id":1,"title":"change","content":"change action"}' -p eosio
executed transaction: 8cb561a712f2741560118651aefd49efd161e3d73c56f6d24cf1d699c265e2dc 224 bytes 2130 us

eosio <= eosio::change {"user":"eosio","post_id":1,"title":"change","content":"change action"}

下面我们看一下数据库：

cleos get table eosio eosio data
{
"rows": [{

  "post_id": 0,
  "poster": "eosio",
  "title": "first",
  "content": "eostea create a first one"
},{
  "post_id": 1,
  "poster": "eosio",
  "title": "change",
  "content": "change action"
},{
  "post_id": 2,
  "poster": "eostea",
  "title": "eostea first",
  "content": "eostea create a first one"
}

],
"more": false
}
post_id=1的记录已经被改变，说明我们成功了。

删除数据

删除数据我又加了一个action,如下所示:

void test_da::dele(account_name user, uint64_t post_id)

{
    require_auth(user);
    das datable( _self, user);
    auto post = datable.find(post_id);
    eosio::print(post->title.c_str());

    eosio_assert(post->poster == user, "yonghucuowu");
    datable.erase(post);
}

这里调用了erase方法删除数据，参数为一个数据对象。下面我们来看一下执行结果：

cleos push action eosio dele '{"user":"eosio","post_id":1}' -p eosioexecuted transaction: 3affbbbbd1da328ddcf37753f1f2f6c5ecc36cd81a0e12fea0c789e75b59714e 200 bytes 2383 us

eosio <= eosio::dele {"user":"eosio","post_id":1}

现在再来看一下我们的数据库：

cleos get table eosio eosio data
{
"rows": [{

  "post_id": 0,
  "poster": "eosio",
  "title": "first",
  "content": "eostea create a first one"
},{
  "post_id": 2,
  "poster": "eostea",
  "title": "eostea first",
  "content": "eostea create a first one"
}

],
"more": false
}
post_id=1的数据已经被我们删除。

到这里，数据库的增删改查已经讲解完毕。