Joins Query

models.rb

Campaign 
has_many :campaign_offers

CampaignOffer
has_many :campaign_offer_reports
belongs_to :campaign

CampaignOfferReport
belongs_to :campaign_offer

query.rb

Campaign.joins(:campaign_offers, :campaign_offer_reports).select("campaigns.*","sum(campaign_offer_reports.profit) as cor_profit").group("campaigns.id")

query.sql

SELECT campaigns.*, sum(campaign_offer_reports.profit) as cor_profit FROM `campaigns` INNER JOIN `campaign_offers` ON `campaign_offers`.`campaign_id` = `campaigns`.`id` INNER JOIN `campaign_offers` `campaign_offers_campaigns_join` ON `campaign_offers_campaigns_join`.`campaign_id` = `campaigns`.`id` INNER JOIN `campaign_offer_reports` ON `campaign_offer_reports`.`campaign_offer_id` = `campaign_offers_campaigns_join`.`id` GROUP BY campaigns.id

so what I get with this query is a table with campaign details and the profit of all reports of that campaign.
But I also want to display the campaigns without campaign_offers/campaign_offer_reports so profit should be then displayed as null

Use left join instead of inner join.

select().from("campaigns left join campaign_offers on campaigns.id = campaign_offer.campaign_id left join campaign_reports on campaigns.id = campaign_reports.campaign_id")