derivation-filter-transform

Stateless Estuary derivation that filters documents and/or transforms field values. Each source document produces zero or one output document; no state, no cross-document coordination.

Prereq: read derivation-basics first for concepts, project layout, workflow, and language choice.

When to use this over alternatives

• Filtering (WHERE clause): drop documents that don't match criteria
• Computed fields: derive new fields from existing ones (CASE, arithmetic, string manipulation, lookups)
• Data cleansing: normalise formats, trim whitespace, replace nulls
• Selecting a subset of fields when combined with the above

For a plain rename or nested-field flattening with no computation, use schema-projections — a projection doesn't require a derivation task.

Canonical Example — SQL

Cleanse a customers collection for downstream analytics: drop inactive accounts and those with obviously invalid emails, normalise the email, and compute a tier bucket from lifetime spend.

schema.yaml

The writeSchema (what the lambda must emit). Enum constraints on tier are enforced at write time; the readSchema (in flow.yaml below) layers in inference for everything else. See derivation-basics § Schema setup.

type: object
properties:
  customer_id:
    type: string
  email_normalized:
    type: string
  signup_date:
    type: string
    format: date
  tier:
    type: string
    enum: [bronze, silver, gold]
required: [customer_id, email_normalized, tier]

flow.yaml

collections:
  acmeCo/analytics/active-customers:
    writeSchema: schema.yaml
    readSchema:
      allOf:
        - $ref: flow://write-schema
        - $ref: flow://inferred-schema
    key: [/customer_id]
    derive:
      using:
        sqlite: {}
      transforms:
        - name: cleanseCustomers
          source: acmeCo/production/customers
          shuffle: any
          lambda: |
            SELECT
              $id AS customer_id,
              LOWER(TRIM($email)) AS email_normalized,
              date($created_at) AS signup_date,
              CASE
                WHEN $lifetime_spend >= 10000 THEN 'gold'
                WHEN $lifetime_spend >= 1000  THEN 'silver'
                ELSE 'bronze'
              END AS tier
            WHERE $status = 'active'
              AND $email LIKE '%@%';

How it works:

• $field binds to the source document's JSON pointer /field. Nested fields use $nested$field, or json_extract($obj, '$.path') for deep paths.
• The WHERE clause drops rows that don't match — they produce zero output documents.
• Any per-row SQLite expression works: CASE, LOWER, TRIM, date(), arithmetic, string concatenation, COALESCE, CAST.

Preview and publish via the workflow in derivation-basics.

Canonical Example — TypeScript

Same task in TypeScript. Reach for TS when the logic is too tangled for SQL: nested parsing, try/catch around dirty data, lookup maps, or splitting one input into many outputs. Here, TS lets us split full_name into first_name / last_name — awkward in SQLite.

schema.yaml

type: object
properties:
  customer_id:
    type: string
  email_normalized:
    type: string
  first_name:
    type: string
  last_name:
    type: string
  signup_date:
    type: string
    format: date
  tier:
    type: string
    enum: [bronze, silver, gold]
required: [customer_id, email_normalized, tier]

flow.yaml

collections:
  acmeCo/analytics/active-customers:
    writeSchema: schema.yaml
    readSchema:
      allOf:
        - $ref: flow://write-schema
        - $ref: flow://inferred-schema
    key: [/customer_id]
    derive:
      using:
        typescript:
          module: cleanse.ts
      transforms:
        - name: cleanseCustomers
          source: acmeCo/production/customers
          shuffle: any

cleanse.ts

Generated with flowctl generate --source flow.yaml, then filled in:

import { IDerivation, Document, SourceCleanseCustomers } from 'flow/acmeCo/analytics/active-customers.ts';

export class Derivation extends IDerivation {
  cleanseCustomers(_read: { doc: SourceCleanseCustomers }): Document[] {
    const c = _read.doc;

    // Filter: return [] to drop the document
    if (c.status !== 'active' || !c.email?.includes('@')) {
      return [];
    }

    const [first, ...rest] = (c.full_name ?? '').trim().split(/\s+/);
    const tier =
      c.lifetime_spend >= 10000 ? 'gold' :
      c.lifetime_spend >= 1000  ? 'silver' : 'bronze';

    return [{
      customer_id: c.id,
      email_normalized: c.email.toLowerCase().trim(),
      first_name: first || '',
      last_name: rest.join(' '),
      signup_date: c.created_at.split('T')[0],
      tier,
    }];
  }
}

Return [] to drop, [{...}] to emit one, [{...}, {...}] to emit many.

Test

Local validation — flowctl preview --fixture (works today)

Write newline-delimited JSON fixtures of source documents and run the derivation locally. Output can be diffed against expected:

fixture.jsonl:

["acmeCo/production/customers", {"id": "c1", "status": "active",   "email": "Alice@Example.COM", "lifetime_spend": 12000, "full_name": "Alice Smith",  "created_at": "2024-01-15T10:00:00Z"}]
["acmeCo/production/customers", {"id": "c2", "status": "active",   "email": "bob@example.com",   "lifetime_spend": 2500,  "full_name": "Bob Jones",    "created_at": "2024-02-01T10:00:00Z"}]
["acmeCo/production/customers", {"id": "c3", "status": "active",   "email": "carol@example.com", "lifetime_spend": 50,    "full_name": "Carol Nguyen", "created_at": "2024-03-10T10:00:00Z"}]
["acmeCo/production/customers", {"id": "c4", "status": "inactive", "email": "dave@example.com",  "lifetime_spend": 5000,  "full_name": "Dave Kim",     "created_at": "2024-04-05T10:00:00Z"}]
["acmeCo/production/customers", {"id": "c5", "status": "active",   "email": "no-at-sign",        "lifetime_spend": 3000,  "full_name": "Eve Watson",   "created_at": "2024-05-20T10:00:00Z"}]
{"commit": true}

flowctl preview --source flow.yaml \
  --name acmeCo/analytics/active-customers \
  --fixture fixture.jsonl --timeout 30s

Expected output (c4 dropped as inactive, c5 dropped for missing @):

{"customer_id":"c1","email_normalized":"alice@example.com","signup_date":"2024-01-15","tier":"gold"}
{"customer_id":"c2","email_normalized":"bob@example.com","signup_date":"2024-02-01","tier":"silver"}
{"customer_id":"c3","email_normalized":"carol@example.com","signup_date":"2024-03-10","tier":"bronze"}

Spec-defined tests — flowctl catalog test

You can also declare tests in-spec with tests:. This is the documented approach and is useful in CI. flowctl preview --fixture is still handy for fast local iteration without round-tripping to the control plane.

tests:
  acmeCo/tests/cleanse-customers:
    - ingest:
        description: Five customers covering each tier, plus an inactive account and an invalid email.
        collection: acmeCo/production/customers
        documents:
          - { id: "c1", status: "active",   email: "Alice@Example.COM", lifetime_spend: 12000, full_name: "Alice Smith",  created_at: "2024-01-15T10:00:00Z" }
          - { id: "c2", status: "active",   email: "bob@example.com",   lifetime_spend: 2500,  full_name: "Bob Jones",    created_at: "2024-02-01T10:00:00Z" }
          - { id: "c3", status: "active",   email: "carol@example.com", lifetime_spend: 50,    full_name: "Carol Nguyen", created_at: "2024-03-10T10:00:00Z" }
          - { id: "c4", status: "inactive", email: "dave@example.com",  lifetime_spend: 5000,  full_name: "Dave Kim",     created_at: "2024-04-05T10:00:00Z" }
          - { id: "c5", status: "active",   email: "no-at-sign",        lifetime_spend: 3000,  full_name: "Eve Watson",   created_at: "2024-05-20T10:00:00Z" }
    - verify:
        description: c4 dropped (inactive) and c5 dropped (no @ in email); remaining three land in their respective tiers with a normalised email.
        collection: acmeCo/analytics/active-customers
        documents:
          - { customer_id: "c1", email_normalized: "alice@example.com", signup_date: "2024-01-15", tier: "gold" }
          - { customer_id: "c2", email_normalized: "bob@example.com",   signup_date: "2024-02-01", tier: "silver" }
          - { customer_id: "c3", email_normalized: "carol@example.com", signup_date: "2024-03-10", tier: "bronze" }

For the TypeScript version, add first_name and last_name to each verify document (e.g. first_name: "Alice", last_name: "Smith").

Variations

• Pass through whole document (filter-only): SELECT JSON($flow_document) WHERE $status = 'active'; — emits each matching source document verbatim at the top level, without listing fields. Estuary's equivalent of SELECT * (which SQLite derivations don't support).
• Preserve deletes through a filter: a WHERE on a business field drops delete events, because a delete carries only the key and _meta (the tested field binds as NULL). To keep hard deletes propagating downstream, exempt deletes from the filter and double-emit them in the same lambda:

  SELECT JSON($flow_document) WHERE $_meta$op != 'd' AND $status = 'active';
  SELECT $id, $_meta WHERE $_meta$op = 'd';
  SELECT $id, $_meta WHERE $_meta$op = 'd';
  

  The delete is emitted twice so the reduction has a prior document to act on; see derivation-basics § Handling deletes for the full mechanism.
• Pass through + add/modify a field: SELECT json_set($flow_document, '$.newField', <expr>); — json_set, json_insert, and json_replace each handle existing/new values differently (https://sqlite.org/json1.html#jins). Needed because SELECT JSON($flow_document), extra_col re-nests the document.
• Filter only, explicit columns: SELECT $id, $name, $email WHERE $status = 'active';
• Extract nested values: $nested$field in SQL, doc.nested?.field in TS. For deep paths in SQL: json_extract($obj, '$.a.b.c').
• Lookup/mapping: in TS, define a Record<string, string> constant and look up values per document.
• NULL handling: COALESCE($field, 'default') in SQL, doc.field ?? 'default' in TS.
• Proper boolean output: SQLite stores booleans as 0/1. Use json('true') / json('false') (often inside a CASE) to emit a real JSON boolean.
• Multi-output per input: both languages support it. TS returns multiple documents from the array; SQL emits one document per output row, so UNION ALL, json_each, or recursive CTEs all work. (For array unnesting specifically, see derivation-flatten-array.)

Gotchas specific to filter/transform

• WHERE is evaluated per document, not per collection. If source documents are updated in place, the filter re-runs on the update. A document that once matched may stop matching — the derivation will emit the new version (which may be empty), not delete the old. Downstream materialisations see the last emitted state.
• Prefer shuffle: any for stateless transforms. Everything in this skill is stateless, so any-shard routing is simplest and avoids unnecessary co-location overhead. Keyed shuffle is not an error here — just wasteful.
• JSON($flow_document) vs $flow_document matters. The JSON() wrapper makes the whole document become the output doc at the top level. Without it, the doc ends up nested under a flow_document property and schema validation fails with "Missing required property: id" — because id is now inside the nested object, not at the root.

Related

• derivation-basics — prerequisite reading
• schema-projections — for plain renames without computation, no derivation needed
• derivation-flatten-array — when one input should produce many outputs (array unnesting)
• derivation-aggregate-metrics — when you need reductions (sum, min, max) across documents
• derivation-join-collections — when combining data across collections