03 — JoinPlan

Status: Draft (protocol v1)
Applies to: Compliance Level 1+

After parsing and validating a query, an implementation MUST produce a JoinPlan (logical name; not required to be a public JSON API). Resolvers consume this plan to fetch only requested data.

Semantic fields

Field	Type	Meaning
rootEntity	string	MeshQL entity key
fields	string[]	Qualified scalar selections for the root (and possibly joins, depending on implementation). The TS planner uses table-prefixed paths such as users.id.
idField	string	Identifying field on the root entity (default id)
joins	ResolvedJoin[]	Nested relations requested by the client
list	object?	Present only for list reads (L2) — see 05
context	object	At least: request correlation, HTTP method, optional entityId for point reads

ResolvedJoin

Field	Type	Meaning
path	string	Dot-separated ref path from root selection, e.g. comments or comments.author
joinKey	string	Schema join key: {parentAstName}.{refName} e.g. post.comments
entity	string	Target entity key
on	string	Join predicate / hint from schema (SQL-like string in the TS reference)
fields	string[]	Selected fields for this join hop
type	"one" | "many"	Cardinality
refName	string	Last segment of path
idField	string	Identifying field of the joined entity (default id)

Example

Query: post { id title comments { id body author { name } } }

Illustrative plan:

{
  "rootEntity": "post",
  "idField": "id",
  "fields": ["posts.id", "posts.title"],
  "joins": [
    {
      "path": "comments",
      "joinKey": "post.comments",
      "entity": "comment",
      "type": "many",
      "refName": "comments",
      "idField": "id",
      "on": "comments.post_id = posts.id",
      "fields": ["comments.id", "comments.body"]
    },
    {
      "path": "comments.author",
      "joinKey": "comments.author",
      "entity": "user",
      "type": "one",
      "refName": "author",
      "idField": "id",
      "on": "users.id = comments.author_id",
      "fields": ["users.name"]
    }
  ]
}

Exact string forms of fields / on may differ by SQL dialect adapters; cardinality, paths, and join keys MUST remain consistent with the query tree.

Point vs list

• Point read: context.entityId is set; list is absent.
• List read: list MAY be set; entityId is absent.

Preshaped resolvers

An implementation MAY skip the shaper if the resolver returns already-nested JSON matching the selection (TS: { preshaped: true }). That behavior is an optimization; the JoinPlan remains the source of truth for what was requested.