使用 ecto 查询连接 3 个表

Posted 2023-04-12

【中文标题】使用 ecto 查询连接 3 个表

【英文标题】：Join 3 tables using ecto query

【发布时间】：2018-03-14 14:04:39

【问题描述】：

这是Postgres的场景

CREATE TABLE "hearings" (
  "hearing_id" int4 NOT NULL,
  "date" timestamp NOT NULL,
  "time" varchar(32),
  "committee" varchar(255),
  "summary" text,
);


CREATE TABLE "categories" (
  "category_id" int4 NOT NULL,
  "description" varchar(255) NOT NULL,
  "type" varchar(32) DEFAULT 'normal' NOT NULL,
);


CREATE TABLE "hearing_category_link" (
  "hearing_id" int4 NOT NULL,
  "category_id" int4 NOT NULL,
);

Postgres 查询：

SELECT * FROM
(hearings LEFT JOIN hearing_category_link ON hearings.hearing_id = hearing_category_link.hearing_id )
LEFT JOIN categories ON hearing_category_link.category_id = categories.category_id

到目前为止，我只能将当前查询与一个表连接起来

from h in "hearings", join: hl in "hearing_category_link", on: hl.hearing_id == h.hearing_id

但我想使用 ecto 加入这 3 个表并获得结果（动态会很棒）。 Another similar question but with postgres

【参考方案1】：

你只需要写2个连接：

from hearing in "hearings",
  join: hcl in "hearing_category_link", on: hcl.hearing_id == hearing.id,
  join: category in "categories", on: hcl.category_id == category.id