11 Column Definitions

A column definition, or def for short, associates a column name, its type and any additional options. It has the form:

(NAME TYPE [OPTIONS...])

name and type are symbols specifying the name and type of the column, respectively (see Octet Molding/Mashing). options are strings passed directly to PostgreSQL as part of a CREATE TABLE command. For example, the status output of the rsync(1) program can be specified by the form:

(define rsync-defs
        '((time            timestamp)
          (error_condition text)
          (files           *text)
          (wrote           int4)          ; bytes
          (read            **text)
          (rate            float4)
          (total           int4)          ; bytes
          (speedup         float4)
          (etc             *int4)))

Likewise, here is an example that might be useful in keeping a table of expenses (although probably using float4 is not a good idea for monetary values):

(define expense-ledger-defs
        '((i       serial)
          (date    timestamp)
          (amount  float4)
          (details *text)))

Note that there are no options in these examples. The components of a def can be extracted with procedures in the postgres-col-defs module, which can be loaded like so:

(use-modules ((database postgres-col-defs)
              #:prefix def:))

In this example, we use the #:prefix clause to systematically prefix def: to the names that the client module would see (resulting in def:column-name and so on).

Procedure: column-name def ¶

Extract column name, a symbol, from def.

Procedure: type-name def ¶

Extract type name, a symbol, from def.

Procedure: type-options def ¶

Extract type options, a list, from def. Each option element is either a string (possibly with embedded spaces), or a sub-list of numbers and/or symbols. Typically the sub-list, if any, will be the first option element.

Procedure: validate-def obj [typecheck] ¶

Check obj and signal error if it does not appear to be a well-formed def. Check that obj has a structure amenable to extraction of components using column-name and type-name: The name must be a symbol using only non-whitespace characters; the type must be a symbol. Optional second arg typecheck is a procedure that takes the type (a symbol) and can do further checks on it. It should return non-#f to indicate success.

There are two more convenience procedures, the first one useful in transforming the results of a query into Scheme objects (see Result Transforms):

Procedure: objectifiers defs ¶

Return a list of objectifiers associated with the types in defs.

Procedure: stringifiers defs ¶

Return a list of stringifiers associated with the types in defs.