Column types and attributes in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the correct syntax for adding a string column with a default value

Which option correctly adds a name column of type string with a default value of 'guest' in a Rails migration?

Ruby on Rails

class AddNameToUsers < ActiveRecord::Migration[7.0]
  def change
    add_column :users, :name, :string, default: 'guest'
  end
end

A add_column :users, :name, :string, default => 'guest'

B add_column :users, :name, :string, default: 'guest'

C add_column :users, :name, :string, default_value: 'guest'

D add_column :users, :name, :string, :default => 'guest'

💡 Hint

Look for the correct keyword argument syntax for default values in Ruby.

In Rails migrations, the default value for a column is set using the keyword argument default:. Option B uses the correct syntax. Option B uses an invalid keyword default_value. Option B uses an incorrect hash rocket syntax without a symbol key. Option B uses a hash rocket but the symbol is passed as a symbol argument instead of a keyword argument, which is invalid here.

❓ component_behavior

intermediate

What is the effect of null: false on a column?

In a Rails migration, what does adding null: false to a column definition do?

Ruby on Rails

add_column :users, :email, :string, null: false

A Makes the column optional in Rails validations

B Allows the column to accept NULL values

C Sets the default value of the column to false

D Prevents the column from accepting NULL values in the database

💡 Hint

Think about database constraints and what null: false means.

The null: false option adds a database-level constraint that disallows NULL values in that column. This means the database will reject any record where this column is NULL. It does not set a default value or affect Rails validations directly.

❓ state_output

advanced

What is the default limit for a string column in Rails migrations?

When you create a string column in a Rails migration without specifying a limit, what is the default maximum length in the database?

Ruby on Rails

add_column :users, :username, :string

A 65,535 characters

B No limit

C 255 characters

D 191 characters

💡 Hint

Consider common database defaults for VARCHAR columns.

Rails maps the string type to a VARCHAR(255) column by default in most SQL databases. This means the maximum length is 255 characters unless you specify a different limit.

🔧 Debug

advanced

Why does this migration raise an error?

Given the migration code below, why does it raise an error when run?

Ruby on Rails

class AddAgeToUsers < ActiveRecord::Migration[7.0]
  def change
    add_column :users, :age, :integer, limit: 10
  end
end

A The limit option for integer columns must be 1, 2, 3, 4, or 8 bytes; 10 is invalid

B The column name 'age' is reserved and cannot be used

C The migration is missing a default value for the age column

D The limit option is only valid for string columns, not integers

💡 Hint

Check the valid byte sizes for integer limits in Rails migrations.

In Rails, the limit option for integer columns specifies the number of bytes and must be one of 1, 2, 3, 4, or 8. Using 10 is invalid and causes an error. The limit option is valid for integers, the column name 'age' is allowed, and default values are optional.

🧠 Conceptual

expert

How does Rails handle decimal column precision and scale?

In a Rails migration, when defining a decimal column with precision and scale, what do these two attributes control?

Ruby on Rails

add_column :products, :price, :decimal, precision: 8, scale: 2

A <code>precision</code> is total digits stored; <code>scale</code> is digits after the decimal point

B <code>precision</code> is digits after the decimal; <code>scale</code> is total digits stored

C <code>precision</code> sets the maximum value; <code>scale</code> sets the minimum value

D <code>precision</code> and <code>scale</code> are synonyms and control total digits

💡 Hint

Think about how numbers with decimals are stored in databases.

In Rails migrations, precision defines the total number of digits that can be stored, including both sides of the decimal point. scale defines how many digits can be stored after the decimal point. For example, precision 8 and scale 2 means up to 6 digits before and 2 digits after the decimal.

Practice

1. Which column type in Rails is best suited to store true or false values?

easy

A. boolean

B. string

C. integer

D. text

Solution

1. Step 1: Understand column types for data

   Boolean columns store true or false values, perfect for yes/no data.

2. Step 2: Compare with other types

   String and text store text, integer stores numbers, so they don't fit true/false data.

3. Final Answer:

   boolean -> Option A

4. Quick Check:

   Boolean = true/false [OK]

Hint: True/false data uses boolean type in Rails [OK]

Common Mistakes:

• Choosing string or text for true/false data
• Using integer to represent boolean values
• Confusing text with boolean type

2. Which of the following is the correct syntax to add a non-nullable string column named title with a default value 'Untitled' in a Rails migration?

easy

A. add_column :books, :title, :text, null: false, default: 'Untitled'

B. add_column :books, :title, :string, default: 'Untitled', null: true

C. add_column :books, :title, :string, null: false, default: 'Untitled'

D. add_column :books, :title, :string, null: false

Solution

1. Step 1: Identify correct column type and options

   We want a string type with null: false and default: 'Untitled'.

2. Step 2: Check syntax order and options

   add_column :books, :title, :string, null: false, default: 'Untitled' correctly uses :string, null: false, and default: 'Untitled'. The option with null: true allows null values. The option with :text uses the wrong type. The option without default misses the default value.

3. Final Answer:

   add_column :books, :title, :string, null: false, default: 'Untitled' -> Option C

4. Quick Check:

   Correct syntax with null and default options [OK]

Hint: Use null: false and default: 'value' together in add_column [OK]

Common Mistakes:

• Forgetting null: false for required columns
• Using wrong column type like text instead of string
• Omitting default value when needed

3. Given this migration snippet:

create_table :users do |t|
  t.integer :age, null: false, default: 18
  t.boolean :active, default: true
end

What will be the default value of active for a new user if not specified?

medium

A. true

B. false

C. nil

D. 0

Solution

1. Step 1: Read the default value for active

   The migration sets default: true for the boolean column active.

2. Step 2: Understand default behavior

   If no value is given when creating a user, active will be set to true automatically.

3. Final Answer:

   true -> Option A

4. Quick Check:

   Boolean default true means new records get true [OK]

Hint: Default value sets initial data if none provided [OK]

Common Mistakes:

• Assuming default is false if not specified
• Confusing nil with default value
• Thinking integer 0 is default for boolean

4. You wrote this migration line:

add_column :orders, :price, :integer, null: true, default: 0

But you want to ensure price is always set and never null. What is the fix?

medium

A. Add null: true, default: nil

B. Change null: true to null: false

C. Change :integer to :float

D. Remove default: 0

Solution

1. Step 1: Identify null constraint

   Currently null: true allows price to be null, which is not desired.

2. Step 2: Fix null constraint

   Changing null: true to null: false ensures price cannot be null.

3. Final Answer:

   Change null: true to null: false -> Option B

4. Quick Check:

   Use null: false to prevent null values [OK]

Hint: null: false means column must have a value [OK]

Common Mistakes:

• Leaving null: true when value must be required
• Removing default without fixing null
• Changing type without addressing null

5. You want to add a status column to your tasks table that stores short text but limits it to 10 characters, defaults to 'pending', and cannot be null. Which migration line is correct?

hard

A. add_column :tasks, :status, :string, limit: 255, null: false, default: 'pending'

B. add_column :tasks, :status, :text, null: false, default: 'pending'

C. add_column :tasks, :status, :string, null: true, default: 'pending'

D. add_column :tasks, :status, :string, limit: 10, null: false, default: 'pending'

Solution

1. Step 1: Choose correct column type and limit

   Use :string with limit: 10 to restrict text length to 10 characters.

2. Step 2: Set null and default options

   Set null: false to require a value and default: 'pending' for initial value.

3. Final Answer:

   add_column :tasks, :status, :string, limit: 10, null: false, default: 'pending' -> Option D

4. Quick Check:

   String with limit, null false, and default set [OK]

Hint: Use limit to restrict string length in migrations [OK]

Common Mistakes:

• Using text type which ignores limit
• Allowing null when value is required
• Setting wrong limit size