OSS重磅推出OSS Select——使用SQL选取文件的内容

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。

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 actionvoid create(account_name user, string title, string content);private:// @abi table data i64struct 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<N(byposter)>();
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。
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原文链接: https://eosfans.io/topics/484

转载于:https://www.cnblogs.com/Weh5211314/p/9055641.html