TableA
has_and_belongs_to_many :table_b

TableB
has_and_belongs_to_many :table_a

TableC is the join table of TableA and TableB

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Arel 2.0.4 (Good query)

SELECT COUNT(DISTINCT "table_a"."id")
FROM "table_a"
LEFT OUTER JOIN "table_c" ON
"table_c"."table_a_id" = "table_a"."id"
LEFT OUTER JOIN "table_b" ON
"table_b"."id" = "table_c"."table_b_id"

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Arel 2.0.5 (Fucked-up query)

SELECT COUNT(DISTINCT "table_a"."id")
FROM "table_a"
LEFT OUTER JOIN "table_c" ON
'table_c' AND "table_c" AND 'table_a_id' AND "table_c"."table_a_id" AND 0 AND "table_a" AND 'id' AND "table_a"."id" AND
"table_c"."table_a_id" = "table_a"."id"
LEFT OUTER JOIN "table_b" ON
0 AND "table_b" AND 'id' AND "table_b"."id" AND 0 AND "table_c" AND 'table_b_id' AND "table_c"."table_b_id" AND
"table_b"."id" = "table_c"."table_b_id"