Covers: - Three-schema approach (overview of Conceptual, Logical and Physical Data Model) - DEMO: Express with raw SQL (pitfalls of not using an ORM) - What good is an Object Relational Mapper (ORM)? - DEMO: Express with Sequelize (models) - What good are migrations? - DEMO: Express with Sequelize (migrations) - Sequelize ecosystem, strengths and weaknesses

1. Sequelize
models, migrations, oh my
Ryan M Harrison Jan 24, 2019

2. Agenda
• Three-schema approach
• DEMO: Express w/ SQL
• What good is an “ORM”?
• DEMO: Express w/ Sequelize (models)
• What good are migrations?
• DEMO: Express w/ Sequelize (migrations)
Follow along in Github:
https://github.com/rmharrison/prez-sequelize
2

3. Three-schema approach
• Conceptual Data Model
• Logical Data Model
• Physical Data Model
3

4. Three-schema approach
4

5. Three-schema approach
5

6. DEMO: Express w/ SQL
• Run postgres in docker
• psql to create table
• psql to seed table
6

7. DEMO: Express w/ SQL
• full_name → first_name, last_name
7

8. Web Framework express, hapi, restify
ORM sequelize, bookshelf
Migration tool umzug (sequelize-cli), knex
(bookshelf)
Query builder sequelize, knex
(Data access object)
Client pg
ORM: Object Relational Mapper (postgres)
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9. ORM: Object Relational Mapper
9
SQL
CREATE TABLE Person (
id bigserial primary key,
full_name varchar(100) NOT NULL,
created_at timestamp default
CURRENT_TIMESTAMP
updated_at timestamp default
CURRENT_TIMESTAMP
);
Sequelize ORM (javascript)
module.exports = (sequelize, DataTypes) => {
const person = sequelize.define('person', {
full_name: DataTypes.STRING,
}, {
underscored: true,
});
return person;
};

10. ORM: Object Relational Mapper
• Virtual Object
• Abstracts (some) implementation details, e.g. associations.
10
SQL
SELECT * FROM people;
SELECT * FROM people
LIMIT 1;
SELECT COUNT(*) FROM people;
SELECT * FROM people WHERE id = 1
Sequelize ORM (javascript)
Person.findAll()
Person.findOne()
Person.findAndCountAll()
Person.findById(1)

11. ORM: Object Relational Mapper
• Virtual Object
• Abstracts (some) implementation details, e.g. associations.
11
Sequelize, One-To-Many associations (hasMany)
const City = sequelize.define('city', { countryCode: Sequelize.STRING });
const Country = sequelize.define('country', { isoCode: Sequelize.STRING });
// Here we can connect countries and cities base on country code
Country.hasMany(City, {foreignKey: 'countryCode', sourceKey: 'isoCode'});
City.belongsTo(Country, {foreignKey: 'countryCode', targetKey: 'isoCode'});

12. Web Framework express, hapi, restify
ORM mongoose
Migration tool migrate-mongoose
Query Builder mongodb
Client mongodb-core
ORM: Object Relational Mapper (mongo)
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13. DEMO: Express w/ Sequelize (models)
• Sequelize CLI
• Generate models from CLI
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14. Migrations: Define schema
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demodb=# d people
Table "public.people"
Column | Type | Collation | Nullable | Default
------------+--------------------------+-----------+----------+-----------------------------------
id | integer | | not null |
nextval('people_id_seq'::regclass)
created_at | timestamp with time zone | | not null | now()
updated_at | timestamp with time zone | | not null | now()
full_name | character varying(255) | | |
Indexes:
"people_pkey" PRIMARY KEY, btree (id)

15. Migrations: Define schema
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demodb=# d people
Table "public.people"
Column | Type | Collation | Nullable | Default
------------+--------------------------+-----------+----------+-----------------------------------
id | integer | | not null |
nextval('people_id_seq'::regclass)
created_at | timestamp with time zone | | not null | now()
updated_at | timestamp with time zone | | not null | now()
first_name | character varying(255) | | |
last_name | character varying(255) | | |
Indexes:
"people_pkey" PRIMARY KEY, btree (id)

16. Migrations: Define schema
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Time Jan 2018 Feb 2018 Mar 2018
release v0.1.0 v0.2.0 v0.3.0
models sales.js
product.js
sales.js
product.js (13 ++++++-----)
store.js
sales.js (100 +++---)
product.js (50 ++++)
store.js (20 +-----)
migrations 201801-migration.js 201802-migration.js
201801-migration.js
201803-migration.js
201802-migration.js
201801-migration.js

17. Seeds: Define data
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demodb=# SELECT * FROM people;
id | created_at | updated_at | first_name |
last_name
----+----------------------------+----------------------------+------------+-----------
1 | 2019-01-25 09:20:11.392+00 | 2019-01-25 09:20:11.392+00 | John | Watts
2 | 2019-01-25 09:20:11.392+00 | 2019-01-25 09:20:11.392+00 | Charles | Ingram

18. DEMO: Express w/ Sequelize (migrations)
• full_name → first_name, last_name
• Generate migrations and seeds from CLI
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19. Strengths
● Model-driven development
● Sequelize Models are Javascript classes
○ all the goodies available, inc. Prototype, extends, etc.
● Life-cycle hooks
○ beforeSave, afterValidate, et al.
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20. Weaknesses addressed ecosystem
● Generate models from existing DB schema
○ npm:sequelize-auto (1400+ stars; officially supported)
● Generate migrations from models
○ Long-standing known issue: https://github.com/sequelize/cli/issues/157
○ npm:sequelize-auto-migrations (incomplete, only handles up)
● Model versioning
○ npm:sequelize-version (low adoption)
● Nested hierarchies
○ npm:sequelize-hierarchy
● Fixtures
○ npm:sequelize-fixtures (maintainer wants to abandon)
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21. Gotchas
● Cannot link a seed with a specific migration
○ Convention: Assume “latest” for seed data
○ Check SequelizeMeta in seed script
● “Version” is not what you think
○ NB: version in Sequelize is for OptimisticLocking [PR] not model versions
● Poor error messages in migrations
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== 20190125055636-full-to-first-last: migrating =======
ERROR: Unexpected identifier

22. Resources
● SQL Naming Conventions
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23. Questions?