has_many :through with polymorphic source and source_type not filtering by memberable_type (related to #205)

[apacosta]

Description

The fix in PR #206 (for #205) resolved the polymorphic type filtering when traversing from a polymorphic parent (e.g., Company → members → users). However, we're seeing the same issue in the reverse direction — traversing to a polymorphic target via source and source_type (e.g., User → members → projects).

Viewing User → Projects in Forest Admin joins members on memberable_id but omits WHERE memberable_type = 'Project', so records from all memberable types show up.

Environment

• forest_admin_rails 1.24.x (also reproduced on 1.16.10)
• Rails 7.x, Ruby 3.x, PostgreSQL

Reproduction

Schema

create_table "members" do |t|
  t.string "memberable_type", null: false
  t.bigint "memberable_id", null: false
  t.bigint "user_id", null: false
  t.timestamps
  t.index ["user_id", "memberable_type", "memberable_id"], unique: true
end

Models

class Member < ApplicationRecord
  belongs_to :user, inverse_of: :members
  belongs_to :memberable, polymorphic: true
end

class User < ApplicationRecord
  has_many :members, dependent: :destroy, inverse_of: :user

  # source_type filters by memberable_type — this is the broken direction
  has_many :projects,  through: :members, source: :memberable, source_type: "Project"
  has_many :teams,     through: :members, source: :memberable, source_type: "Team"
  has_many :companies, through: :members, source: :memberable, source_type: "Company"
end

class Project < ApplicationRecord
  has_many :members, as: :memberable, dependent: :destroy
  has_many :users, through: :members, source: :user  # ← this direction was fixed by #206
end

Data

user = User.create!(name: "Test User")
project = Project.create!(name: "My Project")
team = Team.create!(name: "My Team")

Member.create!(user: user, memberable: project)  # memberable_type = "Project"
Member.create!(user: user, memberable: team)      # memberable_type = "Team"

Expected

User → Projects tab shows 1 record (the project).

Rails generates the correct query:

SELECT "projects".* FROM "projects"
INNER JOIN "members" ON "members"."memberable_id" = "projects"."id"
                    AND "members"."memberable_type" = 'Project'
WHERE "members"."user_id" = 1

Actual

User → Projects tab shows incorrect results — the memberable_type = 'Project' condition is missing from the query, so records from all memberable types leak through.

Relationship to #205

#205 fixed the Polymorphic Parent → through → target direction (e.g., Company → members → users). This appears to be the inverse: Source → through → polymorphic target using source_type (e.g., User → members → projects). It looks like the source_type option on has_many :through isn't being picked up when Forest Admin builds the relationship query.

Happy to provide any additional details or help test a fix. Thanks for your time!