API with NestJS #16. Using the array data type with PostgreSQL and TypeORM

November 2, 2020

This entry is part 16 of 121 in the API with NestJS

Storing arrays is not an obvious thing in the world of SQL databases. Solutions such as MySQL, MariaDB, or Microsoft SQL Server don’t have a straightforward column type for arrays.

This article explores how the array data type works in PostgreSQL both through SQL queries and through TypeORM. By learning how to operate on arrays through SQL, we can better understand what the Postgres database is capable of. This will helps us quite a bit in using arrays through TypeORM.

The capabilities of the array data type in Postgres

Because databases such as MySQL don’t have an array data type, we might have had to work around this issue. One solution would be to create additional tables to store data that we would conceptualize as an array. Another solution would be to utilize the JSON data type available in MySQL and PostgreSQL.

To define the array data type column, we can append the square brackets or use the ARRAY keyword. Let’s play with our post table a bit and add the paragraphs column instead of content.

ALTER TABLE post

DROP COLUMN content,

ADD COLUMN paragraphs text[]

ALTER TABLE post

DROP COLUMN content,

ADD COLUMN paragraphs text ARRAY

The text is a column data type that stores strings of any length

Although we could provide the array’s size, it does not affect the behavior of the database and might only serve as documentation.

When defining the array value input, we can use the curly braces notation or use the ARRAY keyword again.

1 2	INSERT INTO post(title, paragraphs, "authorId") VALUES ('Hello world!', '{"Lorem ipsum", "Dolor sit amet"}', 1);

Please note that we’ve surrounded the curly braces with single quotes and used the double quotes for strings

1 2	INSERT INTO post(title, paragraphs, "authorId") VALUES ('Hello world!', ARRAY['Lorem ipsum', 'Dolor sit amet'], 1);

I lean more towards using the ARRAY keyword, and therefore, it is more frequently used in this article.

An important note is that PostgreSQL will keep the elements in the array in the order you’ve put them in.

Modifying an array

We can change an existing array, for example, by replacing it as a whole.

UPDATE post

SET paragraphs = ARRAY['Lorem ipsum', 'Dolor sit amet']

WHERE id = 1

Another option is to update a single element.

UPDATE post

SET paragraphs[1] = 'Lorem ipsum'

WHERE id = 1

We can also update just a slice of an array. The following code updates the second and the third element, leaving the first element unchanged:

UPDATE post

SET paragraphs[2:3] = ARRAY['Hello', 'World!']

WHERE id = 1

PostgreSQL also supports concatenation with the use of the || operator. The following code creates a new array using the elements 1,2,5 and 6.

UPDATE post

SET paragraphs = paragraphs[1:2] || paragraphs[5:6]

WHERE id = 1

Searching through arrays

When searching through arrays, the ANY and ALL keywords can be very helpful.

To find a post where all paragraphs are equal to 'Apples', we can use the ALL command.

1 2	SELECT * FROM post WHERE 'Apples' = ALL (paragraphs)

To look for a post when any paragraph equals Oranges, we can use the ANY keyword.

1 2	SELECT * FROM post WHERE 'Oranges' = ANY (paragraphs)

Multi-dimensional arrays

PostgresSQL also supports multi-dimensional arrays. To define them, we can use multiple square brackets.

1	ADD COLUMN twodimensional integer[][]

Using them is similar to the regular arrays.

1	SET twodimensional = ARRAY[[0,1], [2,3]]

If you would like to use multi-dimensional arrays and need more examples, look at the official documentation.

Changing a regular column into an array

Above, we’ve completely dropped the content column. This might not be the best approach in production because it would result in data loss. Instead, let’s set the value of content to be the first element of the paragraphs array.

A simple way to achieve that is to:

add the paragraphs column
set its first element to be the value of the content column
remove the content array

ALTER TABLE post

ADD COLUMN paragraphs text ARRAY;

UPDATE post

SET paragraphs = ARRAY[content];

ALTER TABLE post

DROP COLUMN content;

Keep in mind that PostgreSQL utilizes a one-based numbering convention. It means that the array starts with index number 1, not 0.

Using arrays with TypeORM

While knowing all of the above helps us understand what arrays can be used, our goal is to implement them with TypeORM and NestJS. To define a column that is an array, we need to add the array: true property.

import { Column, Entity } from 'typeorm';

@Entity()

class Post {

@Column('text', { array: true })

public paragraphs: string[];

// ...

}

export default Post;

Since we expect our users to send an array of text, we also need to change our Data Transfer Objects and their validation. To check if the property is an array of strings, we need the @IsString({ each: true }) decorator.

import { IsString, IsNotEmpty } from 'class-validator';

export class CreatePostDto {

@IsString({ each: true })

@IsNotEmpty()

paragraphs: string[];

// ...

}

Doing the above is enough to start creating posts with the paragraphs array.

If you want to know how the author property is created in the above response, check out API with NestJS #3. Authenticating users with bcrypt, Passport, JWT, and cookies

When we look into the database, we can see that the paragraphs have been inserted properly.

1 2	SELECT * FROM post WHERE id = 10

Running more advanced queries on arrays

Since arrays are not a common data type in SQL databases, TypeORM might not support all of the features that we’ve used through regular SQL queries.

Fortunately, we can squeeze bare SQL queries into our NestJS code.

async getPostsWithParagraph(paragraph: string) {

return this.postsRepository

.query(`SELECT * from post WHERE ${paragraph} = ANY(paragraphs)`);

}

There is an issue with the above code, though. Here, we are constructing a raw SQL query using a parameter that might be provided by a user. This opens up us for a SQL injection.

Fortunately, with TypeORM, we can create a parameterized query. In the below example, the $1 would be replaced with a value of a paragraph.

async getPostsWithParagraph(paragraph: string) {

return this.postsRepository

.query('SELECT * from post WHERE $1 = ANY(paragraphs)', [paragraph]);

}

The simple-array column type

A side note for creating array columns with TypeORM is that we are not completely out of luck if we don’t use PostgreSQL.

TypeORM has a special simple-array column type that uses a regular string column under the hood.

1 2	@Column("simple-array") paragraphs: string[];

Even though TypeORM exposes the values as an array, it uses a single string column under the hood. All values are separated using a comma, so we can’t have any commas in our values.

Summary

In this article, we’ve gone through the concept of arrays in PostgreSQL both through writing SQL queries and using TypeORM. Thanks to knowing how to deal with arrays through SQL, we could better integrate them into our NestJS code. Fortunately, TypeORM allows us to write SQL queries ourselves, so the knowledge of Postgres really can come in handy.

Series Navigation<< API with NestJS #15. Defining transactions with PostgreSQL and TypeORMAPI with NestJS #17. Offset and keyset pagination with PostgreSQL and TypeORM >>

4 Comments

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Lorenzo

3 years ago

Hello,
Nice post.
What about Jsonb postgresql feature with Nestjs ?
Thanks

Pratik Kulkarni

Hello,

How would we append data to an array of objects here?

Thank you.

azim

how to push array column with typeorm

Stephanie

2 years ago

There’s a slight trick with updatePost.dto.ts, most times the update I just as createPost but with an id, so nestjs has implemented this way to solve it. It can be used in any resource, update.dto.

// if using swagger
import { PartialType } from ‘@nestjs/swagger’;
otherwise
import { PartialType } from ‘@nestjs/mapped-types’;
import CreatePostDto from ‘./createPost.dto’;

export class UpdatePostDto extends PartialType(CreatePostDto) {}
export default UpdatePostDto;

Last edited 2 years ago by Stephanie