Rails - includes, joins & references

2017-05-31 ·

TL;DR

  • joins: just filtering results - not accessing records from a relationship
  • includes: when accessing relationship between records

Lazy loading is a way to defer initialization of an object until it is needed. It may provide additional efficiency in the program's operation, especially if such initialization is costly, e.g. accessing network services or database. The opposite of lazy loading is eager loading.

In Rails includes uses eager loading whereas joins uses lazy loading. With the former both tables are loaded into memory - this reduces the amount of queries required to retrieve associated data.

@companies = Company.includes(:persons).where(persons: { active: true } ).all

@companies.each do |company|
  company.person.name
end

Without the eager loading mechanism iterating through companies would generate a separate database query in order to retrive person's name.

If you don't plan to reference any data from the associated Person table, you can use joins to load it lazy minimizing the required memory utilization. Otherwise, joins may trigger N+1 quries.

@companies = Company.joins(:persons).where(persons: { active: true } ).all

@companies.each do |company|
  company.name
end

includes just loads the data eagrly; at times you may also need the functionality that joins provides i.e. using the related table in where or order by, in that case you need to additionally use references.

# Will work
Company.includes(:persons).references(:persons).where('persons.age > 30')
Company.joins(:persons).where('persons.age > 30')

# Will NOT work
Company.references(:persons).where('persons.age > 30')
Company.includes(:persons).where('persons.age > 30')
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