references

Use to indicate that the given table_names are referenced by an SQL string, and should therefore be +JOIN+ed in any query rather than loaded separately. This method only works in conjunction with #includes. See #includes for more details.

User.includes(:posts).where("posts.name = 'foo'")
# Doesn't JOIN the posts table, resulting in an error.

User.includes(:posts).where("posts.name = 'foo'").references(:posts)
# Query now knows the string references posts, so adds a JOIN