Why Array-contains queries in Firebase? - Purpose & Use Cases

The Big Idea

What if you could find any item inside a list instantly, without flipping through every page?

The Scenario

Imagine you have a list of friends and each friend has a list of hobbies stored in a big notebook. Now, you want to find all friends who like "soccer". You start flipping through every page, reading each hobby list one by one.

The Problem

This manual search is slow and tiring. You might miss some friends or make mistakes. If the notebook grows bigger, it takes even longer. Doing this by hand is frustrating and wastes time.

The Solution

Array-contains queries let you ask the notebook directly: "Show me all friends whose hobbies include 'soccer'." The system quickly finds the right pages without reading everything, saving time and avoiding errors.

Before vs After

✗ Before

for friend in friends:
    if 'soccer' in friend.hobbies:
        print(friend.name)

✓ After

db.collection('friends').where('hobbies', 'array-contains', 'soccer').get().then(querySnapshot => {
  querySnapshot.forEach(doc => {
    console.log(doc.id, ' => ', doc.data());
  });
});

What It Enables

You can instantly find all records containing a specific item inside an array, making searches fast and simple.

Real Life Example

A social app wants to show users who like a certain sport or music genre. Using array-contains queries, it quickly finds and displays those users without scanning the entire database.

Key Takeaways

Manual searching through arrays is slow and error-prone.

Array-contains queries let you search inside arrays efficiently.

This makes your app faster and your code cleaner.

Practice

1.

What does the array-contains query operator do in Firebase Firestore?

easy

A. Finds documents where an array field includes a specific value

B. Finds documents where a string field contains a substring

C. Finds documents where a number field is greater than a value

D. Finds documents where a field is missing

Solution

1. Step 1: Understand the purpose of array-contains

   This operator is designed to check if an array field in a document contains a specific value.

2. Step 2: Compare with other query types

   Other options describe different query operators, but only array-contains matches arrays including a value.

3. Final Answer:

   Finds documents where an array field includes a specific value -> Option A

4. Quick Check:

   array-contains = finds value in array [OK]

Hint: Remember: array-contains checks for one value inside an array [OK]

Common Mistakes:

• Confusing array-contains with substring search
• Using array-contains on non-array fields
• Thinking it works with multiple values at once

2.

Which of the following is the correct syntax to query documents where the tags array contains the value "urgent"?

const query = firestore.collection('tasks').where(____);

easy

A. "tags", "contains", "urgent"

B. "tags", "==", "urgent"

C. "tags", "array-contains-any", ["urgent"]

D. "tags", "array-contains", "urgent"

Solution

1. Step 1: Recall the correct operator for single value in array

   The operator is "array-contains" to check if an array includes a specific value.

2. Step 2: Check syntax format

   The syntax is .where("field", "array-contains", value). "tags", "array-contains", "urgent" matches this exactly.

3. Final Answer:

   "tags", "array-contains", "urgent" -> Option D

4. Quick Check:

   Correct syntax uses array-contains [OK]

Hint: Use "array-contains" for single value array queries [OK]

Common Mistakes:

• Using == instead of array-contains for arrays
• Confusing array-contains with array-contains-any
• Using non-existent operator "contains"

3.

Given the following documents in a Firestore collection projects:

[{ id: 1, members: ["alice", "bob"] }, { id: 2, members: ["carol"] }, { id: 3, members: ["bob", "dave"] }]

What will the query firestore.collection('projects').where('members', 'array-contains', 'bob').get() return?

medium

A. Documents with id 2 only

B. Documents with id 1 only

C. Documents with id 1 and 3

D. No documents

Solution

1. Step 1: Identify documents where 'members' array contains 'bob'

   Document 1 has members ["alice", "bob"] which includes "bob". Document 3 has ["bob", "dave"] which also includes "bob". Document 2 does not.

2. Step 2: Determine query result

   The query returns documents 1 and 3 because their 'members' arrays contain "bob".

3. Final Answer:

   Documents with id 1 and 3 -> Option C

4. Quick Check:

   array-contains 'bob' = docs 1 and 3 [OK]

Hint: Check which arrays include the value exactly [OK]

Common Mistakes:

• Including documents without the value
• Confusing array-contains with array-contains-any
• Assuming partial matches inside strings

4.

What is wrong with this Firestore query?

firestore.collection('orders').where('items', 'array-contains', ['apple', 'banana'])

The items field is an array of strings.

medium

A. You cannot use an array as the value with array-contains

B. The operator should be 'array-contains-any' instead

C. The field 'items' must be a string, not an array

D. The query syntax is correct

Solution

1. Step 1: Understand array-contains value type

   The 'array-contains' operator expects a single value, not an array, to check inside the array field.

2. Step 2: Identify correct usage

   Passing an array as the value causes an error. To check multiple values, 'array-contains-any' is used instead.

3. Final Answer:

   You cannot use an array as the value with array-contains -> Option A

4. Quick Check:

   array-contains needs single value, not array [OK]

Hint: array-contains accepts only one value, not arrays [OK]

Common Mistakes:

• Passing an array as the value to array-contains
• Confusing array-contains with array-contains-any
• Assuming array-contains works like 'in' operator

5.

You want to find all documents in the events collection where the attendees array contains both "john" and "jane". Which approach is correct?

hard

A. Use a single query with array-contains-any and pass both names

B. Use two queries with array-contains for each name and merge results client-side

C. Use a single query with array-contains and pass an array of both names

D. Use a query with where('attendees', '==', ['john', 'jane'])

Solution

1. Step 1: Understand limitations of array-contains and array-contains-any

   array-contains checks for one value in array; array-contains-any checks if array contains any of multiple values but not all.

2. Step 2: Find correct method to get documents containing both values

   Since Firestore does not support querying for multiple values all present in one array in a single query, you must run two queries with array-contains and combine results client-side.

3. Final Answer:

   Use two queries with array-contains for each name and merge results client-side -> Option B

4. Quick Check:

   Multiple array-contains queries combined client-side [OK]

Hint: Firestore can't query multiple array-contains in one query [OK]

Common Mistakes:

• Using array-contains-any expecting all values matched
• Passing array of values to array-contains
• Using equality check on arrays