Replace ComputationDetail with a function adapter registry

[myronmarston]

Prep refactor #2 of 2 paving the way for #1327 (approximatePercentile). This does not add the percentile function — it makes the framework ready for it. Closes #1330.

Stacked on #1331 — review that one first.

Why

ElasticGraph treats aggregated value functions uniformly, expressing differences via runtime metadata rather than special casing. That works only while every function takes no arguments and returns a flat {"value" => ...} response.

A function that needs a request argument and returns a nested response has nowhere for that knowledge to live, so it leaks as if function == :percentiles conditionals across three files: the clause builder, the resolver, and the empty-bucket builder.

After this change, adding a function is a registry entry plus one adapter — no changes to any of those three.

The adapter interface

Method	Responsibility
datastore_function_name	The datastore's aggregation type (may differ from the ElasticGraph function name)
extract_args(args, element_names)	GraphQL args → canonically-keyed args hash
clause_options(function_args)	Extra keys merged into the aggregation clause alongside field
extract_result(raw)	Locate the value hash within the datastore's response
empty_bucket_result	The complete fabricated response for a bucket the datastore omitted

All five existing functions differ only in datastore name and empty-bucket value, so one SimpleMetric data class parameterized on those two covers all of them; its other three methods are no-ops. A function whose behavior isn't a parameterization of anything would register as a singleton module instead.

Key decisions

• Metadata names the adapter; the adapter owns the datastore aggregation name. Breaking that coupling lets the two diverge, which the percentile function needs. For all five existing functions the metadata string is unchanged.
• Fields resolve the name to an adapter once, at boot. Fields are built once and cached, so resolution is per field rather than per query, the registry has one query-time reader, and an unregistered name fails at boot rather than mid-query.
• Arguments flow in two phases: extract, then apply. Argument names are customizable via schema element names, which the computation value object can't see when building a clause. Query building calls extract_args at the boundary; clause building later calls the pure clause_options. Threading element names onto the computation would pollute its identity, which is used to dedupe computations in a Set.
• The empty-bucket value moves into the registry. It's fully derivable from the function (verified across all 45 aggregated value fields: sum/cardinality → 0; avg/min/max → nil), so per-field storage was redundant and let an author pair a function with a wrong empty value. The adapter returns the entire fabricated response rather than a bare value the builder must wrap.
• Argument plumbing ships now, though no function uses it yet. The point of these prep tickets is that the framework is ready; deferring would leave the follow-up changing the adapter interface itself.

Renames

Before	After
ComputationDetail	deleted (class, spec, RBS, requires)
GraphQLField#computation_detail	computation_function (a bare Symbol)
GraphQLField#with_computation_detail	deleted
Field#runtime_metadata_computation_detail	computes(function)
Schema::Field#computation_detail	function_adapter
Computation#detail	function_adapter + function_args

Runtime metadata per field collapses from three keys to one:

approximate_sum:
  computation_function: sum

computes :sum reads as part of the field DSL, which is otherwise unprefixed (documentation, mapping, json_schema). It validates the name against the registry at dump time, mirroring how elasticgraph-schema_definition already depends on elasticgraph-graphql to validate scalar coercion adapters — no new gem dependency.

Verification

• script/type_check — clean
• script/lint — 891 files, no offenses
• Full suite — 5227 examples, 0 failures
• Mutation-checked the empty-bucket path: breaking empty_bucket_result fails 2 tests

Note: script/quick_build exits non-zero on SimpleCov's 100%-coverage gate (79 uncovered lines across 16 files, none touched here). Confirmed pre-existing — the base commit fails identically with the same 79 lines.

Follow-up

With both prep changes landed, #1327 reduces to a percentile adapter, one registry entry, the field definition, two schema element names, docs, and tests. No framework changes.

🤖 Generated with Claude Code

[myronmarston]

feedback.

[myronmarston]

We should use a different name here since we plan to add percentile support in a later PR--that would break this test.

[myronmarston]

Suggested change

	# `computes` validates the function name against the query engine's registry at dump time, mirroring
	# how this gem already depends on `elasticgraph-graphql` to validate scalar coercion adapters.

[myronmarston]

Let's just define this as an anonymous singleton object within the example that needs it, rather than as a globally accessible class.