Array-contains queries in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Understanding array-contains query behavior

Given a Firestore collection 'users' where each document has a field 'roles' which is an array of strings, what will the following query return?

db.collection('users').where('roles', 'array-contains', 'admin').get()

Assuming documents:
- User A: roles = ['admin', 'editor']
- User B: roles = ['viewer']
- User C: roles = ['admin']

Which users will be returned by this query?

A Users A, B, and C

B Users B and C only

C User C only

D Users A and C only

💡 Hint

The 'array-contains' operator matches documents where the array field contains the specified value anywhere.

The query filters documents where the 'roles' array contains the string 'admin'. User A has 'admin' in roles, User C has 'admin', but User B does not. So only Users A and C are returned.

🧠 Conceptual

intermediate

Limitations of array-contains queries

Which of the following statements about Firestore's 'array-contains' queries is TRUE?

A 'array-contains' can be used to check if an array contains multiple values at once.

B 'array-contains' can be combined with 'array-contains-any' in the same query.

C 'array-contains' can only check for a single value inside an array field.

D 'array-contains' works on fields that are strings, not arrays.

💡 Hint

Think about what 'array-contains' means: it checks for one value inside an array.

'array-contains' only checks if a single value exists inside an array field. To check multiple values, 'array-contains-any' is used. It cannot be combined with 'array-contains' in the same query.

❓ Configuration

advanced

Correct Firestore query for multiple array values

You want to find documents in a 'products' collection where the 'tags' array contains either 'eco' or 'sale'. Which Firestore query code correctly achieves this?

A db.collection('products').where('tags', 'array-contains-any', ['eco', 'sale']).get()

B db.collection('products').where('tags', 'array-contains', ['eco', 'sale']).get()

C db.collection('products').where('tags', 'in', ['eco', 'sale']).get()

D db.collection('products').where('tags', 'array-contains-all', ['eco', 'sale']).get()

💡 Hint

'array-contains-any' checks if the array contains any of the listed values.

'array-contains-any' allows querying documents where the array field contains at least one of the specified values. 'array-contains' only accepts a single value, 'in' works on field values, not array contents, and 'array-contains-all' does not exist.

❓ security

advanced

Security rules impact on array-contains queries

Consider Firestore security rules that restrict read access to documents where the 'roles' array contains 'admin'. Which rule snippet correctly enforces this for read operations?

A allow read: if 'admin' in resource.data.roles;

B allow read: if resource.data.roles == 'admin';

C allow read: if request.auth.token.roles.array-contains('admin');

D allow read: if resource.data.roles.contains('admin');

💡 Hint

Firestore security rules use JavaScript-like syntax with 'in' to check array membership.

In Firestore security rules, to check if an array contains a value, use the 'in' operator: 'value in array'. Option A correctly uses 'admin' in resource.data.roles. Options A and D use invalid syntax, and B compares array to string.

❓ Architecture

expert

Designing Firestore indexes for array-contains queries

You have a Firestore collection 'orders' with a field 'items' which is an array of product IDs. You want to efficiently query orders containing a specific product ID using 'array-contains'. What is the best practice regarding indexes to support this query?

A Create a composite index on 'items' and 'orderDate' fields to support 'array-contains' queries.

B No index is needed; Firestore automatically indexes array fields for 'array-contains' queries.

C Manually create a single-field index on 'items' to enable 'array-contains' queries.

D Disable indexing on 'items' to improve write performance since 'array-contains' queries do not use indexes.

💡 Hint

Firestore automatically indexes array fields for 'array-contains' queries without extra setup.

Firestore automatically creates single-field indexes on array fields to support 'array-contains' queries. Composite indexes are needed only when combining multiple where clauses or ordering. Disabling indexing on 'items' would break queries.