Rails中的外键-错误

Question

I'm trying to add a new foreign key column to my Customers table. This is my migration:

class AddCompanyForeignKeyToCustomers < ActiveRecord::Migration[5.1]
  def change
    add_reference :customers, :company, foreign_key: true
    add_foreign_key :customers, :companies
  end
end

These are the errors I'm getting:

rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::DuplicateObject: ERROR:  constraint "fk_rails_ef51a916ef" for relation "customers" already exists
: ALTER TABLE "customers" ADD CONSTRAINT "fk_rails_ef51a916ef"
FOREIGN KEY ("company_id")
  REFERENCES "companies" ("id")

ActiveRecord::StatementInvalid: PG::DuplicateObject: ERROR:  constraint "fk_rails_ef51a916ef" for relation "customers" already exists
: ALTER TABLE "customers" ADD CONSTRAINT "fk_rails_ef51a916ef"
FOREIGN KEY ("company_id")
  REFERENCES "companies" ("id")

PG::DuplicateObject: ERROR:  constraint "fk_rails_ef51a916ef" for relation "customers" already exists

What does any of that mean? I don't know what fk_rails_$NUMBER is.

Answer 1

You're trying to add foreign keys twice by calling add_reference and add_foreign_key . The docs will tell you more, but basically add_foreign_key adds a key and add_reference can add a foreign key (which you are doing with foreign_key: true in your code above), so you should just use one or the other.

If I were you I would just use add_foreign_key, it seems more suited to what you're trying to do:

class AddCompanyForeignKeyToCustomers < ActiveRecord::Migration[5.1]
  def change
    add_foreign_key :customers, :companies
  end
end

Answer 2

It means that you are trying to create a foreign key constraint that duplicates one you already have.

This line creates a foreign key constraint on customers referencing companies , as well as creates the company_id column

add_reference :customers, :company, foreign_key: true

This line is trying to do the foreign key constraint again

add_foreign_key :customers, :companies

Just remove one or the other, depending on whether or not you also need to add the company_id column