IK
Там поля ему важны, а не наличие fk/pk
irb(main):005:0> PSerializer.new(P.includes({ as: :c }).first, { include: [:as, :'as.c'] }).serialized_json
P Load (0.2ms) SELECT "ps".* FROM "ps" ORDER BY "ps"."id" ASC LIMIT ? [["LIMIT", 1]]
A Load (0.1ms) SELECT "as".* FROM "as" WHERE "as"."p_id" = ? [["p_id", 1]]
C Load (0.1ms) SELECT "cs".* FROM "cs" WHERE ("cs"."p_id" = 1 AND "cs"."feature" = 'feature-0' OR "cs"."p_id" = 1 AND "cs"."feature" = 'feature-1' OR "cs"."p_id" = 1 AND "cs"."feature" = 'feature-2' OR "cs"."p_id" = 1 AND "cs"."feature" = 'feature-3' OR "cs"."p_id" = 1 AND "cs"."feature" = 'feature-kek')
=> "{\"data\":{\"id\":\"1\",\"type\":\"p\",\"relationships\":{\"as\":{\"data\":[{\"id\":\"1\",\"type\":\"a\"},{\"id\":\"2\",\"type\":\"a\"},{\"id\":\"3\",\"type\":\"a\"},{\"id\":\"4\",\"type\":\"a\"},{\"id\":\"5\",\"type\":\"a\"}]}}},\"included\":[{\"id\":\"1\",\"type\":\"a\",\"relationships\":{\"p\":{\"data\":{\"id\":\"1\",\"type\":\"p\"}},\"c\":{\"data\":{\"id\":\"1\",\"type\":\"c\"}}}},{\"id\":\"2\",\"type\":\"a\",\"relationships\":{\"p\":{\"data\":{\"id\":\"1\",\"type\":\"p\"}},\"c\":{\"data\":{\"id\":\"2\",\"type\":\"c\"}}}},{\"id\":\"3\",\"type\":\"a\",\"relationships\":{\"p\":{\"data\":{\"id\":\"1\",\"type\":\"p\"}},\"c\":{\"data\":{\"id\":\"3\",\"type\":\"c\"}}}},{\"id\":\"4\",\"type\":\"a\",\"relationships\":{\"p\":{\"data\":{\"id\":\"1\",\"type\":\"p\"}},\"c\":{\"data\":{\"id\":\"4\",\"type\":\"c\"}}}},{\"id\":\"5\",\"type\":\"a\",\"relationships\":{\"p\":{\"data\":{\"id\":\"1\",\"type\":\"p\"}},\"c\":{\"data\":null}}},{\"id\":\"1\",\"type\":\"c\",\"relationships\":{\"p\":{\"data\":{\"id\":\"1\",\"type\":\"p\"}}}},{\"id\":\"2\",\"type\":\"c\",\"relationships\":{\"p\":{\"data\":{\"id\":\"1\",\"type\":\"p\"}}}},{\"id\":\"3\",\"type\":\"c\",\"relationships\":{\"p\":{\"data\":{\"id\":\"1\",\"type\":\"p\"}}}},{\"id\":\"4\",\"type\":\"c\",\"relationships\":{\"p\":{\"data\":{\"id\":\"1\",\"type\":\"p\"}}}}]}"