Update: this can now be defined in fieldMatch #216

It is possible for an (invalid) Data Package to have discrepancies between the schema and the actual data. E.g. defining more/less columns or in a different order. read_resource() will silently let those through when the data types of the switched columns are compatible, which can lead to issues for the user (e.g. lat/lon are silently switched). Only when the data types are incompatible, will readr return a parsing issue.

To avoid passing these issues silently, read_resource() should compare the headers of the file with the schema and raise an error if those are not exactly the same. This implements the following spec:

The field descriptor MUST contain a name property. This property SHOULD correspond to the name of field/column in the data file (if it has a name). As such it SHOULD be unique (though it is possible, but very bad practice, for the data file to have multiple columns with the same name). name SHOULD NOT be considered case sensitive in determining uniqueness. However, since it should correspond to the name of the field in the data file it may be important to preserve case.

Implementation considerations:

• Only compare when replace_null(dialect$header, TRUE) (i.e. it is not false). It might be useful to define dialect_header and reuse it here:

  frictionless-r/R/read_resource.R

  Line 356 in 421c22f

  skip = ifelse(replace_null(dialect$header, TRUE), 1, 0),

• The specs say that case should NOT be considered, so both the field names and col_names should be lowercased before comparing

• To allow comparison, the header line of the file should be read separately from the main read_delim(). read_lines() could be used, but delim and encoding/locale might have to be passed too.

• A resource can contain multiple files (e.g. observations_1, observations_2). Either all files are read and compared or only the last once, cf. add_resource():

  The last file will be read with readr::read_delim() to create or compare with schema and to set format, mediatype and encoding. The other files are ignored, but are expected to have the same structure and properties.

• On a mismatch (fieldnames, different order, more or less), an error should be returned, similar to check_schema():

  frictionless-r/R/check_schema.R

  Lines 65 to 69 in 421c22f

  	msg = glue::glue(
  	"Field names in `schema` must match column names in data:",
  	"\u2139 Field names: `{field_names_collapse}`",
  	"\u2139 Column names: `{col_names_collapse}`",
  	.sep = "\n"

• Add a section validation to explain what we validate:

  #' @section Validation:
  #' Full validation is not supported.
  #' Something about validation issues
  #' Something about header compare