A Popular Example of Metaprogramming: ORMs

gistfile1.txt

// Used in the article: https://www.gimtec.io/articles/metraprogramming-example-orm/
const pg = require('pg');

/**
 * ORM Setup with metaprogramming
 */
const define = async (dbUrl, table) => {
  try {
    const getTableSchema = (table) => `select * from INFORMATION_SCHEMA.COLUMNS where TABLE_NAME='${table}';`;
    const client = new pg.Client(dbUrl);
    let isConnected = false;
    const connectClient = () => {
      if (!isConnected) {
        client.connect();
        isConnected = true;
      }
    }

    connectClient();
    const schema = await client.query(getTableSchema(table));
    const rows = schema.rows.map(({ column_name }) => column_name);
    const Model = function (attrs) {
      rows.forEach(row => {
        this[row] = attrs[row];
      });
    }

  
    const createGetBy = (prop) => {
      Model[`getBy${prop.charAt(0).toUpperCase() + prop.slice(1)}`] = async function (value) {
        connectClient();
        const result = await client.query(`select * from ${table} where ${prop} = ${value}`);
        return new Model(result.rows[0]);
      }
    }
  
    Model.getAll = async function () {
      connectClient();
      const result = await client.query(`select * from ${table};`);
      return result.rows.map((row) => new Model(row));
    }
  
    Model.close = function() {
      client.end();
    }
  
    rows.forEach(createGetBy);
  
    return Model;
  } catch (e) {
    console.log('in da define error');
    console.log(e);
  }
};

/**
 * ORM usage examples.
 */
const dbUrl = 'postgres://some-url';
const userTest = async () => {
  // const User = define(dbUrl, 'users', ['email', 'id', 'name']);
  try {
    const User = await define(dbUrl, 'users');
    const users = await User.getAll();
    console.log("USERS:")
    console.log(users);
    const user = await User.getById(10);
    console.log("ONE USER:")
    console.log(user);
    User.close();
  } catch (e) {
    console.log('in da error');
    console.log(e);
    User.close();
  } finally {
  }
};
userTest();

const postTest = async () => {
  // const Post = define(dbUrl, 'posts', ['id', 'user_id', 'title', 'content']);
  const Post = await define(dbUrl, 'posts');
  try {
    const posts = await Post.getAll();
    console.log("POSTS:")
    console.log(posts);
    const post1 = await Post.getById(1);
    console.log("ONE POST:");
    console.log(post1);
    const postsUser = await Post.getByUser_id(11);
    console.log("ONE POST:");
    console.log(postsUser);
  } catch (e) {
    console.log('in da error');
    console.log(e);
  } finally {
    Post.close();
  }
};
postTest();

/**
 * Utils used to setup the database for testing purposes:
 */
const clearTable = 'DELETE FROM users;';
const createUserTable = 'CREATE TABLE users (email varchar, id integer, name varchar);';
const createPostTable = 'CREATE TABLE posts (id integer, user_id integer, title varchar, content varchar);';
const manageTable = () => {
  const client = new pg.Client(dbUrl);
  const insertUserQuery = (user) => {
    return `insert into users (email, id, name) values ('${user.email}', ${user.id}, '${user.name}')`; 
  };
  const insertPostQuery = (post) => {
    return `insert into posts (id, user_id, title, content) values (${post.id}, ${post.userId}, '${post.title}', '${post.content}')`; 
  };
  client.connect();
  // client.query(insertUserQuery({
  //   email: 'amy@test.com',
  //   id: 10,
  //   name: 'Amy',
  // }))
  // client.query(insertPostQuery({
  //   id: 2,
  //   title: 'Test Post 2',
  //   userId: 11,
  //   content: 'This is a test post',
  // }))
  client.query(getTableSchema('users'))
    .then((data) => {
      console.log(data.rows.map((row) => row.column_name));
      client.end();
    });
};
// manageTable();