Input
Path: GET /rest.php/wikibase/v0/entities/lexemes/{id}
Path parameter: Lexeme ID e.g. L123
Query params: none, for now (_fields could be added in a future story)
Headers: conditional request headers (if-none-match, ...), authorization

Response body
id: stays as it is
lemmas: structure gets "flattened", same as the structure of labels and descriptions (language code -> value)
lexical_category: item id value, name changed to use an underscore instead of camel case lexicalCategory.
language: item id value
statements: same as the response structure of the item/property endpoints
forms:

• id: stays as it is
• representations: structure gets "flattened", same as the structure of labels and descriptions (language code -> value)
• statements: same as the response structure of the item/property endpoints
• grammatical_features: list of item id values, name changed to use an underscore instead of camel case

senses:

• id: stays as it is
• statements: same as the response structure of the item/property endpoints
• glosses: structure gets "flattened", same as the structure of labels and descriptions (language code -> value)

Response headers

• ETag for revision ID
• Last-Modified contains last modified date

Response status
200 for success
404 for not found
400 for invalid id

ADR: Principles to follow when creating the lexeme REST API

Task breakdown

• register GET /rest.php/wikibase/v0/entities/lexemes/{id}
  • route handler + use case boilerplate
  • initial e2e test
• enable dev routes on beta wikidata
  • requires a patch in the integration/config repository
  • load routes.dev.json file, similar to what already happens for the wikibase dev routes
• move statement read models and serializers that we need into a statements domain
  • allow CRUD and lexeme to depend on this domain in the corresponding namespaces (arch test updates)
  • update cross domain architecture test (if necessary)
• add lemmas field
• add lexical_category and language field
• add statements field (only for the lexeme, not yet for forms/senses)
• add forms field (might be multiple tickets)
• add senses field (might be multiple tickets)
• add ETag and Last-Modified response headers
• validate lexeme id path param
• handle lexeme not found
• handle lexeme redirects
• handle unexpected exceptions
• create RouteHandlersTest similar to what we have for CRUD/search domain routes