When working with database queries in Active Record, it’s essential to understand the precise behavior of query methods to avoid unexpected results and performance issues.

Key considerations:

Method Override Behaviors: Some methods replace previous configurations rather than augmenting them. For example, pluck ignores any previous select clauses:

# This ignores the select(:email) - only id is selected
Customer.select(:email).pluck(:id)
# => SELECT "customers"."id" FROM customers

# For raw SQL in pluck, use Arel.sql:
Customer.pluck(Arel.sql("DISTINCT id"))
# => SELECT DISTINCT id FROM customers

Query Merging Complexity: When combining queries with merge, or, and and, be careful about how conditions are combined to avoid incorrect SQL generation:

# Complex queries can generate unexpected SQL if not carefully constructed
base = Comment.joins(:post).where(user_id: 1).where("recent = 1")
base.merge(base.where(draft: true).or(Post.where(archived: true)))

Index Usage Control: For performance optimization, understand how to control which indexes are used in queries:

# When using implicit_order_column with multiple columns
# Adding nil at the end prevents appending the primary key
add_index :users, [:created_at, :id], name: "optimal_recent_users_query"
User.implicit_order_column = [:created_at, nil] # Prevents adding id twice to ORDER BY

Carefully understanding these behaviors helps you write more predictable, efficient database queries and avoid unexpected results when refactoring.

Add Repository

Private Repository