Rails ActiveRecord where or clause

One way is to revert to raw sql...

YourModel.where("categories.id IN ? OR category_relationships.category_id IN ?", category_ids, category_ids)

Keep the SQL out of it and use ARel, like this:

.where(Category.arel_table[:id].in(category_ids).  or(CategoryRelationship.arel_table[:category_id].in(category_ids))

Assuming you want to return Categories, you need to OUTER JOIN category_relationships and put a OR condition on the combined table.

Category.includes(:category_relationships).where("categories.id IN (?) OR category_relationships.category_id IN (?)",category_ids,category_ids )

This query is creating an outer join table by combining columns of categories and category_relationships. Unlike an inner join (e.g. Category.joins(:category_relationships)), outer join table would also have categories with no associated category_relationship. It would then apply the conditions in whereclause on the outer join table to return the matching records.

includes statement without conditions on the association usually makes two separate sql queries to retrieve the records and their association. However when used with conditions on the associated table, it would make a single query to create an outer join table and run conditions on the outer join table. This allows you to retrieve records with no association as well.

See this for a detailed explanation.