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。

目录

1. 增
2. 查
3. 改
4. 删

新增

新增内容每每用到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; });　//数据库内容建立 }

这里须要注意的是：

1. 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，数据库存储在谁的帐户下。
2. 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。

那么咱们如今来看一下咱们的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

1. 全部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。
2. 下面是对create函数的声明。
3. 接下来是对数据字段的定义。这里咱们定义了数据结构da.
4. primary_key函数是定义主键的函数。　
5. 接下来咱们定义了辅助主键返回poster。
6. EOSLIB_SERIALIZE宏的第一个参数是数据结构，其余参数是数据结构中的数据成员。
7. typedef咱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。这里咱们定义的是一个具备主键及一个辅助键的数据库对象。
   .abi　

abi 很是重要，错误的abi会致使合约执行失败。

{ "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:

1. 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","...

1. 建立数据

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"}

下面咱们来查一下数据表：

cleos get table eosio eosio data { "rows": [{ "post_id": 0, "poster": "eosio", "title": "first", "content": "create a first one" } ], "more": false }

建立信息成功，那么咱们用别的帐号建立会怎样呢？

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，相信你们对建立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，若是没有查询功能，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，一方面是经过二级索引查询。
这里为了使表中数据多样化我会作一些修改，将全部数据都合到一张表中。
我将以上.cpp中的das datable( _self, user);改成das datable( _self, _self);.这样数据都存在合约帐户的表中。

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文件也作了相应的调整：

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以下：

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，其余查询就不在一一介绍。

执行getd操做：

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

数据库中数据以下：

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。
这里咱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代码以下：

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.　前几行代码已经以前讲解过，如今直接说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,其余的不用多说。
下面咱们执行一下命令：

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"}

下面咱们看一下数据库：

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，说明咱们成功了。

删除数据

删除数据我又加了一个action,以下所示:

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。下面咱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}

如今再来看一下咱们的数据库：

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的数据已经被咱们删除。

到这里，数据库的增删改查已经讲解完毕。
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原文连接: https://eosfans.io/topics/484