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Query API

Querying is how you find records that match certain conditions, for example:

  • Find all comments that belong to a certain post
  • Find all verified comments made by John
  • Count all verified comments made by John or Lucy published under posts made in the last two weeks

Because queries are executed on the database, and not in JavaScript, they're really fast. It's also how Watermelon can be fast even at large scales, because even with tens of thousands of records total, you rarely need to load more than a few dozen records at app launch.

Defining Queries

@children

The simplest query is made using @children. This defines a Query for all comments that belong to a Post:

class Post extends Model {
  // ...
  @children('comments') comments
}

➡️ Learn more: Defining Models

Extended Query

To narrow down a Query (add extra conditions to an existing Query), use .extend():

import { children, lazy } from '@nozbe/watermelondb/decorators'

class Post extends Model {
  // ...
  @children('comments') comments
  @lazy verifiedComments = this.comments.extend(Q.where('is_verified', true))
  @lazy verifiedAwesomeComments = this.verifiedComments.extend(Q.where('is_awesome', true))
}

Note: Use the @lazy when extending or defining new Queries for performance

Custom Queries

You can query any table using this.collections.get(tableName).query(conditions). Here, post.comments will query all users that made a comment under post.

class Post extends Model {
  // ...
  @lazy commenters = this.collections.get('users').query(
    Q.on('comments', 'post_id', this.id)
  )
}

Executing Queries

Most of the time, you connect Queries to Components by using observe or observeCount:

withObservables(['post'], ({ post }) => ({
  post: post.observe(),
  comments: post.comments.observe(),
  verifiedCommentCount: post.verifiedComments.observeCount(),
}))

Fetch

To simply get the current list or current count, use fetch / fetchCount. You might need it in Actions.

const comments = await post.comments.fetch()
const verifiedCommentCount = await post.verifiedComments.fetchCount()

Query conditions

import { Q } from '@nozbe/watermelondb'
// ...
commentCollection.query(
  Q.where('is_verified', true)
)

This will query all comments that are verified (all comments with one condition: the is_verified column of a comment must be true).

When making conditions, you refer to column names of a table (i.e. is_verified, not isVerified). This is because queries are executed directly on the underlying database.

The second argument is the value we want to query for. Note that the passed argument must be the same type as the column (string, number, or boolean; null is allowed only if the column is marked as isOptional: true in the schema).

Empty query

const allComments = await commentCollection.query().fetch()

A Query with no conditions will find all records in the collection.

Note: Don't do this unless necessary. It's generally more efficient to only query the exact records you need.

Multiple conditions

commentCollection.query(
  Q.where('is_verified', true),
  Q.where('is_awesome', true)
)

This queries all comments that are both verified and awesome.

Conditions with other operators

Query JavaScript equivalent
Q.where('is_verified', true) is_verified === true (shortcut syntax)
Q.where('is_verified', Q.eq(true)) is_verified === true
Q.where('archived_at', Q.notEq(null)) archived_at !== null
Q.where('likes', Q.gt(0)) likes > 0
Q.where('likes', Q.weakGt(0)) likes > 0 (slightly different semantics — see "null behavior" for details)
Q.where('likes', Q.gte(100)) likes >= 100
Q.where('dislikes', Q.lt(100)) dislikes < 100
Q.where('dislikes', Q.lte(100)) dislikes <= 100
Q.where('likes', Q.between(10, 100)) likes >= 10 && likes <= 100
Q.where('status', Q.oneOf(['published', 'draft'])) status === 'published' || status === 'draft'
Q.where('status', Q.notIn(['archived', 'deleted'])) status !== 'archived' && status !== 'deleted'
Q.where('status', Q.like('%bl_sh%')) /.*bl.sh.*/g

Note: It's NOT SAFE to use Q.like with user input directly, because special characters like % or _ are not escaped. Always sanitize user input like so: Q.like(\%${Q.sanitizeLikeString(userInput)}%`)`

Conditions on related tables

For example: query all comments under posts published by John:

commentCollection.query(
  Q.on('posts', 'author_id', john.id),
)

Normally you set conditions on the table you're querying. Here we're querying comments, but we have a condition on the post the comment belongs to.

The first argument for Q.on is the table name you're making a condition on. The other two arguments are same as for Q.where.

Note: The two tables must be associated before you can use Q.on.

Advanced Queries

Advanced observing

Call query.observeWithColumns(['foo', 'bar']) to create an Observable that emits a value not only when the list of matching records changes (new records/deleted records), but also when any of the matched records changes its foo or bar column. Use this for observing sorted lists

Count throttling

By default, calling query.observeCount() returns an Observable that is throttled to emit at most once every 250ms. You can disable throttling using query.observeCount(false).

AND/OR nesting

You can nest multiple conditions using Q.and and Q.or:

commentCollection.query(
  Q.where('archived_at', Q.notEq(null)),
  Q.or(
    Q.where('is_verified', true),
    Q.and(
      Q.where('likes', Q.gt(10)),
      Q.where('dislikes', Q.lt(5))
    )
  )
)

This is equivalent to archivedAt !== null && (isVerified || (likes > 10 && dislikes < 5)).

Column comparisons

This queries comments that have more likes than dislikes. Note that we're comparing likes column to another column instead of a value.

commentCollection.query(
  Q.where('likes', Q.gt(Q.column('dislikes')))
)

null behavior

There are some gotchas you should be aware of. The Q.gt, gte, lt, lte, oneOf, notIn, like operators match the semantics of SQLite in terms of how they treat null. Those are different from JavaScript.

Rule of thumb: No null comparisons are allowed.

For example, if you query comments for Q.where('likes', Q.lt(10)), a comment with 8 likes and 0 likes will be included, but a comment with null likes will not! In Watermelon queries, null is not less than any number. That's why you should avoid making table columns optional unless you actually need it.

Similarly, if you query with a column comparison, like Q.where('likes', Q.gt(Q.column('dislikes'))), only comments where both likes and dislikes are not null will be compared. A comment with 5 likes and null dislikes will NOT be included. 5 is not greater than null here.

Q.oneOf operator: It is not allowed to pass null as an argument to Q.oneOf. Instead of Q.oneOf([null, 'published', 'draft']) you need to explicitly allow null as a value like so:

postsCollection.query(
  Q.or(
    Q.where('status', Q.oneOf(['published', 'draft'])),
    Q.where('status', null)
  )
)

Q.notIn operator: If you query, say, posts with Q.where('status', Q.notIn(['published', 'draft'])), it will match posts with a status different than published or draft, however, it will NOT match posts with status == null. If you want to include such posts, query for that explicitly like with the example above.

Q.weakGt operator: This is weakly typed version of Q.gt — one that allows null comparisons. So if you query comments with Q.where('likes', Q.weakGt(Q.column('dislikes'))), it WILL match comments with 5 likes and null dislikes. (For weakGt, unlike standard operators, any number is greater than null).

Next steps

➡️ Now that you've mastered Queries, make more Relations