How to specify conditions on joined tables in rails

Rails ActiveRecord: How to join with conditions

ActiveRecord’s joins and where methods construct powerful SQL queries. The correct usage of these methods is the foundation for crafting complex database fetches in a simple, readable format.

Here's an insider tip: if your association name is different from your table name, always stick to the association name in your joins and the table name in your where clause.

// This doesn't look like rocket science but mind the details!
Submission.joins(:tasks).where(submissions: { task_id: params[:task_id] })
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Solving typical join issues: A troubleshooter’s guide

Complex queries often face their fair share of issues. Access this essential toolkit to prevent common errors and ensure your join operations go off without a hitch:

• Missing Columns: Double-check the existence of 'task_id' in the 'submissions' table.
• Naming Conventions: When referencing tables in Rails, adhere to convention i.e., use :submissions when joining tables and submissions in where clauses.
• Complex Conditions: Implement merge to streamline conditions on joined tables while also improving code readability.
• Test SQL Distinctly: Debug complex scenarios by direct testing of SQL commands in your database console.

Drilling into advanced join queries

Leveraging Associations and Scopes

Named scopes within your associations can effectively handle detailed conditions in your join queries. You employ a scope recently_active for Post:

class Post < ApplicationRecord
  // This isn't magic, it's Rails!
  scope :recently_active, -> { where('updated_at > ?', 1.week.ago) }
end
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This simplifies your join query:

// Aren't you "active" to use this?
User.joins(:posts).merge(Post.recently_active)
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Resolving Ambiguities

If multiple tables contain the same column name, use ModelClassName.table_name to avoid ambiguous column errors:

// Avoiding ambiguities like avoiding spoilers of your favorite show
User.joins(:posts).where(posts: { id: User.table_name + '.post_id' })
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Debugging 101

Use .to_sql to inspect the SQL code generated by ActiveRecord and debug effectively.