• Crow
2021-01-27 19:15:50

http://www2.cs.uregina.ca/~bernatja/crowsfoot.html

Crow’s Foot Notation

A number of data modeling techniques are being used today. One of the most common is the entity relationship diagram (ERD). Several ERD notations are available. For the purposes of CS270 we will be using Crow’s Foot Notation[b1].

Components used in the creation of an ERD:

Entity – A person, place or thing about which we want to collect and store multiple instances of data. It has a name, which is a noun, and attributes which describe the data we are interested in storing. It also has an identifier, which uniquely identifies one instance of an entity. The attribute which acts as the identifier is marked with an asterisk.

Relationship – Illustrates an association between two entities. It has a name which is a verb. It also has cardinality and modality.

Cardinality and Modality are the indicators of the business rules around a relationship. Cardinality refers to the maximum number of times an instance in one entity can be associated with instances in the related entity. Modality refers to the minimum number of times an instance in one entity can be associated with an instance in the related entity.

Cardinalitycan be 1 or Many and the symbol is placed on the outside ends of the relationship line, closest to the entity, Modality can be 1 or 0 and the symbol is placed on the inside, next to the cardinality symbol. For a cardinality of 1 a straight line is drawn. For a cardinality of Many a foot with three toes is drawn. For a modality of 1 a straight line is drawn. For a modality of 0 a circle is drawn.

zero or more [b4]

1 or more [b5]

1 and only 1 (exactly 1) [b6]

zero or 1 [b7]

Cardinality and modality are indicated at both ends of the relationship line. Once this has been done, the relationships are read as being 1 to 1 (1:1), 1 to many (1:M), or many to many (M:M).

1:1

1:M

M:M

1:M

Typically, ERDs are much more complex than this, involving quite a number of entities and relationships. If we join all of the above relationships together and add a few attributes, a small collection of data might be depicted in the following way using Crow’s Foot Notation:

Consider the following business rules for a patient appointment system:

A doctor can be scheduled for many appointments, but may not have any scheduled at all. Each appointment is scheduled with exactly 1 doctor. A patient can schedule 1 or more appointments. One appointment is scheduled with exactly 1 patient. An appointment must generate exactly 1 bill, a bill is generated by only 1 appointment. One payment is applied to exactly 1 bill, and 1 bill can be paid off over time by several payments. A bill can be outstanding, having nothing yet paid on it at all. One patient can make many payments, but a single payment is made by only 1 patient. Some patients are insured by an insurance company. If they are insured, they can only carry insurance with one company. An insurance compnay can have many patients carry their policies. For patients that carry insurance, the insurance company will make payments, each single payment is made by exactly 1 insurance company.

Given the above information, the following ERD can be drawn:

Intersection Entities

Intersection entities are used in the resolution of a many to many relationship. This resolution is done in order to store additional information which doesn’t fit into the attribute list of either entity in the M:M relationship. For instance, in the instructor-course example given earlier, there is a M:M relationship between Course and Section. A course can have many sections, and a section can have many courses. If we want to know how many students are registered in a particular class, we cannot store the information on either entity. We could store the total number of students registered in all sections of a particular course, in the course entity, ie: 300 students are registered in CS270, all sections included. We could store the total number of students registered in all courses with a particular section number, in the section entity, ie: 25,000 students are registered in a section numbered 002. If we want more practical information, such as how many students are registered in section 002 of CS270, we need another entity in which to store the information. So, we create an intersection entity, and place it between its related entities with the additional attributes. The M:M relationship becomes two 1:M relationships. See resolution below.

Independent Entities

Independent entities are those which can exist without another entity. Student, seat, instructor etc. are examples of independent entities.

Dependent Entities

Dependent entities rely upon the existence of other entities. Some of their attributes are taken from the identifiers in the entities upon which they depend. Class is an example of a dependent entity. It relies upon course and section, and gets two of its attributes from their identifiers.

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# Crow的安装与构建

## 前言

项目地址:

https://github.com/crowcpp/crow

官方网站及文档说明:

https://crowcpp.org/master/

缘由:

前几天一直在寻找一款C++的Web框架，通过不断的在Github、StackOverFlow以及搜索引擎上搜索，最终我还是选择了Crow这一框架，其他相关框架还有，restbed,served,nrgest,微软的C++ REST SDK等,Github相关搜索C++ REST framework…

结果在测试用例构建时候，似乎遇到了一个bug，于是我提交了一个Issues,得到了开发人员的回复，最终问题得到了解决，在此，再次感谢项目维护人员的付出！

## 配置环境

本篇文章为Linux安装。其他平台或具体最低要求等，还请见官方说明文档

• Ubuntu 20.0.4
• GCC 9.4.0
• Boost 1.71
• ASIO 1.22.1
• Cmake 3.24.0

到目前为止，Corw依赖使用的ASIO已经为脱离BOOST的独立版。

相关安装参考文章:

相关问题解决参考文章:

Unable to locate package ——Linux新手入门：Unable to locate package错误解决办法

## 安装Crow

从源码安装，详见官方说明文档

git clone https://github.com/CrowCpp/Crow.git


进入Crow目录，创建build目录,并进入。

mkdir build
cd build

cmake .. -DCROW_BUILD_EXAMPLES=OFF -DCROW_BUILD_TESTS=OFF

make install


## Hello world

#include <crow.h>

int main(){
crow::SimpleApp app;
//测试
CROW_ROUTE(app, "/test")([](){
return "Hello world";
});
}


编译

• g++
g++ main.cpp -o  main -lpthread

• CMake
# 指定版本
cmake_minimum_required(VERSION 3.24)
# 项目名
project(main)
# 使用指定源文件来生成目标可执行文件
# 引入外部依赖包
find_package(Crow REQUIRED)
# 目标文件与库文件进行链接


测试

## 我的第一个程序

  CROW_ROUTE(app, "/low_price_recommend").methods("POST"_method)([](const crow::request& req){
crow::json::wvalue return_ret({});//返回结果
if (!x){//参数不全
return_ret["status_code"] = std::string("0");
return crow::response(return_ret);
}else{
return_ret = myfounction.low_price_recommend(
x["flight_date"].s(),
x["flight_departure"].s(),
x["flight_arrival"].s(),
x["flight_agent"].s(),
x["flight_retCounts"].s());
return crow::response(return_ret);
}
});

• myfounction为我封装的一个类创建的对象，low_price_recommend为其中的一个方法。返回一个crow::json::wvalue。
• 变量x为具体传进来的参数
• crow::json::wvalue为返回的json格式数据
通过x[“”].s()将对应的参数转成string

## 可能出现的问题

• CMake 找不到asio库，例如如下所示:
CMake Error at /usr/local/share/cmake-3.24/Modules/CMakeFindDependencyMacro.cmake:47 (find_package):
By not providing "Findasio.cmake" in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by "asio", but
CMake did not find one.

Could not find a package configuration file provided by "asio" with any of
the following names:

asioConfig.cmake
asio-config.cmake

Add the installation prefix of "asio" to CMAKE_PREFIX_PATH or set
"asio_DIR" to a directory containing one of the above files. If "asio"
provides a separate development package or SDK, be sure it has been
installed.
Call Stack (most recent call first):
/usr/local/lib/cmake/Crow/CrowConfig.cmake:32 (find_dependency)
CMakeLists.txt:6 (find_package)

-- Configuring incomplete, errors occurred!

• 解决
• 打开并编辑这个文件/usr/local/lib/cmake/Crow/CrowConfig.cmake

大概在30行左右，将这行

list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake)  替换为 list(APPEND CMAKE_MODULE_PATH${CROW_CMAKE_DIR})


再次感谢Crow项目维护成员Luca Schlecker老哥的解答，以及所有维护此项目的成员。

官方GITTER讨论聊天室——crowfork/community

## 解决跨域问题

#include <crow.h>
#include "query/query.h"
#include "common/common.h"
#include "crow/middlewares/cors.h"

int main(){

crow::App<crow::CORSHandler> app;
auto& cors = app.get_middleware<crow::CORSHandler>();
cors
.global()
.origin("*")
.headers("origin, x-requested-with, accept, access-control-allow-origin, authorization, content-type")
.methods("POST"_method, "GET"_method, "PUT"_method, "DELETE"_method, "PATCH"_method, "OPTIONS"_method);

//测试
CROW_ROUTE(app, "/hello")([](){
return "Hello world";
});
}


展开全文
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# 目录

1. 搭建C++开发环境
2. 安装crow所需依赖
3. 测试

# 1.搭建C++开发环境

安装以下工具

yum -y install gcc
yum -y install gcc-c++
yum -y install gdb
yum -y install git
yum -y install cmake
yum -y install vim*


测试hello world程序

hello.cpp:

#include <stdio.h>
int main(){
printf("hello World\n");
return 0;
}


编译运行：

# 2.安装安装crow所需依赖

项目地址：https://github.com/ipkn/crow
克隆到本地：git clone https://gitclone.com/github.com/ipkn/crow.git
crow依赖于boost library，可以进行编译安装，或者选择简单的方法：yum install boost boost-devel，若系统缺少openssl，也需安装yum install openssl openssl-devel

# 3.测试

首先需要搭建一个C++项目，这样可以使用cmake命令帮助我们编译

mkdir crowTest
cd crowTest

mkdir bin
mkdir lib
mkdir src
mkdir include
mkdir build
touch CMakeLists.txt


将前文中下载的 crow_all.h 文件放入include文件夹中，同时在src下新建 hello.cpp 文件，内容参考crow项目中的自带示例：

hello.cpp:

#include "crow_all.h"

int main()
{
crow::SimpleApp app;

CROW_ROUTE(app, "/")([](){
return "Hello world\n";
});

}


CMakeLists.txt 中的内容如下：

project(crowTest)

set(EXECUTABLE_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/bin) set(LIBRARY_OUTPUT_PATH${PROJECT_SOURCE_DIR}/lib)
include_directories(${PROJECT_SOURCE_DIR}/include) set(BOOST_ROOT "/usr/include/boost") set(BOOSTROOT "/usr/include/boost") include_directories(/usr/include) link_directories(/usr/lib) FIND_PACKAGE(Threads REQUIRED) SET(Boost_USE_MULTITHREADED ON) find_package(Boost REQUIRED COMPONENTS system) add_executable(crowTest src/hello.cpp) target_link_libraries(crowTest boost_thread boost_system${CMAKE_THREAD_LIBS_INIT})


1-4行设置项目相关信息，接下来设置boost相关信息，需要根据boost的安装位置来设置

完成后进行编译：

cd build

cmake ..
make


编译成功后进入项目的bin目录下，启动：

正式环境下我们需要使用nohup在后台运行，这里是为了方便测试，我们可以在另一个会话中进行测试：

测试成功

对于已有程序，可以结合crow，对外提供服务。

展开全文

...