Calling count on an ActiveRecord::Relation with more than 1 distinct attribute in the select clause doesn't honor the select conditions. For example, this works:

User.select('distinct username').all    #=> SELECT distinct username FROM "users"
User.select('distinct username').count  #=> SELECT COUNT(distinct username) FROM "users"

However, the distinct clause disappears from the count SQL when another distinct attribute is specified:

User.select('distinct username, email').all     #=> SELECT distinct username, email FROM "users"
User.select('distinct username, email').count   #=> SELECT count(*) FROM "users"

Same thing with distinct users.*:

User.select('distinct users.*').all     #=> SELECT distinct users.* FROM "users"
User.select('distinct users.*').count   #=> SELECT count(*) FROM "users"

That means for a relation with multiple distinct columns, relation.count is not necessarily the same as relation.all.count.

For distinct users.*, count(:distinct => true) works (it generates SELECT COUNT(DISTINCT "users"."id") FROM "users"), but that won't work for distinct username, email.

I've verified this behavior on Postgres and SQLite.