Scopes for reusable queries in Ruby on Rails - Step-by-Step Execution

Concept Flow - Scopes for reusable queries

Define Scope Method

↓

Call Scope on Model

↓

Scope Returns Relation

↓

Chain or Execute Query

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Get Filtered Results

Scopes are methods defined in a model that return filtered queries. You call them to get reusable, chainable query parts.

Execution Sample

Ruby on Rails

class Product < ApplicationRecord
  scope :available, -> { where(available: true) }
end

Product.available

Defines a scope named 'available' that filters products where available is true, then calls it.

Execution Table

Step	Action	Evaluation	Result
1	Define scope :available	scope :available, -> { where(available: true) }	Scope method added to Product
2	Call Product.available	Product.available	Returns ActiveRecord::Relation with where(available: true)
3	Execute query	Product.available.to_a	Array of products with available = true
4	Chain scopes (if any)	Product.available.other_scope	Combined query with both filters
5	End	No more calls	Query ready for use or execution

💡 Scopes return relations until executed; execution fetches filtered records.

Variable Tracker

Variable	Start	After 1	After 2	Final
Product.available	undefined	ActiveRecord::Relation (where available: true)	Same relation	Array of filtered products after execution

Key Moments - 2 Insights

Why does calling Product.available not immediately fetch records?

Because scopes return a relation object that builds the query but delays execution until needed, as shown in execution_table step 2 and 3.

Can scopes be chained together?

Yes, scopes return relations that can be chained to combine filters, as shown in execution_table step 4.

Visual Quiz - 3 Questions

Look at the execution table, what does Product.available return at step 2?

A An array of products

B An ActiveRecord::Relation with a where clause

C A boolean value

D A SQL string

💡 Hint

Check the 'Evaluation' and 'Result' columns at step 2 in the execution_table.

At which step does the query actually fetch records from the database?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for when the relation is converted to an array in the execution_table.

If you chain another scope after Product.available, what happens to the query?

A It combines filters into one query

B It executes the query immediately

C It replaces the previous scope

D It causes an error

💡 Hint

See step 4 in the execution_table about chaining scopes.

Concept Snapshot

Scopes are reusable query methods in Rails models.
Define with scope :name, -> { query }.
Calling a scope returns a relation, not records.
Scopes can be chained to combine filters.
Query runs when you fetch records (e.g., to_a, each).

Full Transcript

In Rails, scopes let you write reusable query parts inside your model. You define a scope with a name and a lambda that returns a filtered query. When you call the scope, it returns an ActiveRecord relation object, which builds the query but does not run it yet. You can chain multiple scopes to combine filters. The actual database query runs only when you fetch records, like converting to an array or iterating. This makes your code cleaner and queries reusable.