SQLite BLOB images

The shipped SQLite example derives one provider from bridge declarations. Its documents.payload column also shows how the same SQL value can cross a context boundary as an opaque handle or as a structural MeTTa expression.

attach_sqlite declares the default explicitly:

crm = metta.space("&crm")
crm.image("Blob", "opaque")

The resulting atom (image &crm Blob opaque) lives in &metta and has type ImageDecl. Image policy is per type and per context. Another attached SQLite space can therefore choose transparent for Blob without changing &crm. The _ type name supplies a fallback for types a context does not name.

Read one field without projecting the payload

SQLite returns the binary column as the example's Blob object. Under the opaque image, TableBridge carries that object as one grounded handle. A lazy path runs after the surrounding document pattern matches and encodes only the byte it reaches:

payload = bytes(range(64)) * 4
provider.connection.execute(
    "INSERT INTO documents VALUES (?, ?)",
    ("manual", sqlite3.Binary(payload)),
)

rows = metta.space("&crm").match(
    S.document(S.manual, path("data", 17, to=V.byte))
)
assert rows.to_dicts() == [{"byte": 17}]

The acceptance test replaces Blob.__metta__ with a function that raises, then runs this query. It still answers, proving the complete structural image was not requested.

Choose the structural image

Pass blob_image="transparent" when attaching a second context:

transparent = attach_sqlite(m, "&archive", blob_image="transparent")

The same row then answers with (Blob 0 1 2 ...). The acceptance test measures both crossings with space.stats().inferences, takes the minimum of three samples for each, and requires the transparent crossing to cost more than the opaque handle for the same 4,096-byte payload. auto delegates the choice to the standard constant-time size and replayability policy.

See metta.tables for the generic bridge and lazy paths for attribute and key traversal.