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https://github.com/chaudinh/graphql-server-tutorial
https://github.com/chaudinh/graphql-server-tutorial
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- Host: GitHub
- URL: https://github.com/chaudinh/graphql-server-tutorial
- Owner: ChauDinh
- Created: 2019-12-20T13:35:16.000Z (almost 5 years ago)
- Default Branch: master
- Last Pushed: 2023-01-05T03:24:36.000Z (almost 2 years ago)
- Last Synced: 2023-03-03T14:42:29.970Z (over 1 year ago)
- Language: JavaScript
- Size: 533 KB
- Stars: 0
- Watchers: 2
- Forks: 0
- Open Issues: 19
-
Metadata Files:
- Readme: readme.md
Awesome Lists containing this project
README
# Database models, schemas, and entities in postgresql
- Database schema: a database schema is close to the implementation and tells the databases how an entity looks like in a database table, whereas every instance of an entity represented by a table row.
Example: The schema (./src/models/user.js) defines fileds (eg. username) and relationships (eg. a user has messages) of an entity (User). Each field represented a column in database. Basically, the schema is the blueprint for an entity.
- Database model: a database model is more abstract perspective on the schema. If offers developers the conceptual framework on what models are available and how to use models as interfaces to connect a database. Often models are implemented by ORMs.
- Database entity: A database entity is an actual instance of a stored item in the database created by database schema. Each entity uses a row in table whereas each fields of the entity is defined by a column.
- Additional methods: We can also add methods on our models. For example:
```js
const user = (sequelize, DataTypes) => {
const User = sequelize.define("user", {
username: {
type: DataTypes.STRING,
unique: true
}
});
User.associate = models => {
User.hasMany(models.Message);
};
User.findByLogin = async login => {
let user = await User.findOne({ where: { username: login } });
if (!user) {
user = await User.findOne({ where: { email: login } });
}
return user;
};
};
```
The message model looks like similar. You can check both in the paths `./src/models/user.js` and `./src/models/message.js`.
# Connecting Resolvers and Database
What we will cover?
- Use the new models in GraphQL resolvers
- Seed your database with data when the application start
- Add a user model method for retrieving a user by username
- Learn essentials about psql command line.
# Setting Headers for graphql playground
# Offset/limit pagination
The offset based pagination isn't too difficult to implement. The limit states how many items you want to retrieve from the entire list, and the offset states where to begin in the whole list.
The disadvantages of offset/limit pagination is when offset becomes very long, the database query takes longer, leading to a poor client-side performance. In addition, when an item deleted, offset/limit pagination cannot handle this. For instance, if you query the first page of messages and someone deletes a message, the offset/limit would be wrong on the next page since the item count is off by one.
# Cursor-based pagination
We can avoid these disadvantages of offset/limit in cursor pagination.
The cursor-based pagination implements an identifier called cursor rather counting items like offset/limit. The cursor can be used to express "give me a limit of X items from cursor Y".
In this tutorial, the approach is to use dates (eg. the creation date entity in database) to identify an item in the list.