class CreateColumn(_DDLCompiles):
Represent a _schema.Column
as rendered in a CREATE TABLE statement,
via the .CreateTable
construct.
This is provided to support custom column DDL within the generation
of CREATE TABLE statements, by using the
compiler extension documented in :ref:`sqlalchemy.ext.compiler_toplevel`
to extend .CreateColumn
.
Typical integration is to examine the incoming _schema.Column
object, and to redirect compilation if a particular flag or condition
is found:
from sqlalchemy import schema from sqlalchemy.ext.compiler import compiles @compiles(schema.CreateColumn) def compile(element, compiler, **kw): column = element.element if "special" not in column.info: return compiler.visit_create_column(element, **kw) text = "%s SPECIAL DIRECTIVE %s" % ( column.name, compiler.type_compiler.process(column.type) ) default = compiler.get_column_default_string(column) if default is not None: text += " DEFAULT " + default if not column.nullable: text += " NOT NULL" if column.constraints: text += " ".join( compiler.process(const) for const in column.constraints) return text
The above construct can be applied to a _schema.Table
as follows:
from sqlalchemy import Table, Metadata, Column, Integer, String from sqlalchemy import schema metadata = MetaData() table = Table('mytable', MetaData(), Column('x', Integer, info={"special":True}, primary_key=True), Column('y', String(50)), Column('z', String(20), info={"special":True}) ) metadata.create_all(conn)
Above, the directives we've added to the _schema.Column.info
collection
will be detected by our custom compilation scheme:
CREATE TABLE mytable ( x SPECIAL DIRECTIVE INTEGER NOT NULL, y VARCHAR(50), z SPECIAL DIRECTIVE VARCHAR(20), PRIMARY KEY (x) )
The .CreateColumn
construct can also be used to skip certain
columns when producing a CREATE TABLE. This is accomplished by
creating a compilation rule that conditionally returns None.
This is essentially how to produce the same effect as using the
system=True argument on _schema.Column
, which marks a column
as an implicitly-present "system" column.
For example, suppose we wish to produce a _schema.Table
which skips
rendering of the PostgreSQL xmin column against the PostgreSQL
backend, but on other backends does render it, in anticipation of a
triggered rule. A conditional compilation rule could skip this name only
on PostgreSQL:
from sqlalchemy.schema import CreateColumn @compiles(CreateColumn, "postgresql") def skip_xmin(element, compiler, **kw): if element.element.name == 'xmin': return None else: return compiler.visit_create_column(element, **kw) my_table = Table('mytable', metadata, Column('id', Integer, primary_key=True), Column('xmin', Integer) )
Above, a .CreateTable
construct will generate a CREATE TABLE
which only includes the id column in the string; the xmin column
will be omitted, but only against the PostgreSQL backend.
Method | __init__ |
Undocumented |
Class Variable | __visit_name__ |
Undocumented |
Instance Variable | element |
Undocumented |
Inherited from _DDLCompiles
:
Method | _compile_w_cache |
Undocumented |
Method | _compiler |
Return a compiler appropriate for this ClauseElement, given a Dialect. |
Class Variable | _hierarchy_supports_caching |
disable cache warnings for all _DDLCompiles subclasses. |
Inherited from ClauseElement
(via _DDLCompiles
):
Method | compare |
Compare this _expression.ClauseElement to the given _expression.ClauseElement . |
Method | compile |
Compile this SQL expression. |
Method | params |
Return a copy with _expression.bindparam elements replaced. |
Method | self_group |
Apply a 'grouping' to this _expression.ClauseElement . |
Method | unique_params |
Return a copy with _expression.bindparam elements replaced. |
Method | __bool__ |
Undocumented |
Method | __getstate__ |
Undocumented |
Method | __invert__ |
Undocumented |
Method | __repr__ |
Undocumented |
Method | __str__ |
Undocumented |
Method | _clone |
Create a shallow copy of this ClauseElement. |
Method | _cloned_set |
Return the set consisting all cloned ancestors of this ClauseElement. |
Method | _execute_on_connection |
Undocumented |
Method | _negate |
Undocumented |
Method | _negate_in_binary |
a hook to allow the right side of a binary expression to respond to a negation of the binary expression. |
Method | _replace_params |
Undocumented |
Method | _set_propagate_attrs |
Undocumented |
Method | _ungroup |
Return this _expression.ClauseElement without any groupings. |
Method | _with_binary_element_type |
in the context of binary expression, convert the type of this object to the one given. |
Class Variable | _cache_key_traversal |
Undocumented |
Class Variable | _from_objects |
Undocumented |
Class Variable | _is_bind_parameter |
Undocumented |
Class Variable | _is_clause_list |
Undocumented |
Class Variable | _is_clone_of |
Undocumented |
Class Variable | _is_from_clause |
Undocumented |
Class Variable | _is_from_container |
Undocumented |
Class Variable | _is_immutable |
Undocumented |
Class Variable | _is_lambda_element |
Undocumented |
Class Variable | _is_returns_rows |
Undocumented |
Class Variable | _is_select_container |
Undocumented |
Class Variable | _is_select_statement |
Undocumented |
Class Variable | _is_singleton_constant |
Undocumented |
Class Variable | _is_text_clause |
Undocumented |
Class Variable | _is_textual |
Undocumented |
Class Variable | _order_by_label_element |
Undocumented |
Class Variable | bind |
Undocumented |
Class Variable | description |
Undocumented |
Class Variable | is_clause_element |
Undocumented |
Class Variable | is_selectable |
Undocumented |
Class Variable | stringify_dialect |
Undocumented |
Class Variable | supports_execution |
Undocumented |
Instance Variable | _propagate_attrs |
like annotations, however these propagate outwards liberally as SQL constructs are built, and are set up at construction time. |
Property | _constructor |
return the 'constructor' for this ClauseElement. |
Property | entity_namespace |
Undocumented |
Inherited from SQLRole
(via _DDLCompiles
, ClauseElement
):
Class Variable | allows_lambda |
Undocumented |
Class Variable | uses_inspection |
Undocumented |
Inherited from SupportsWrappingAnnotations
(via _DDLCompiles
, ClauseElement
):
Method | _annotate |
return a copy of this ClauseElement with annotations updated by the given dictionary. |
Method | _deannotate |
return a copy of this _expression.ClauseElement with annotations removed. |
Method | _with_annotations |
return a copy of this ClauseElement with annotations replaced by the given dictionary. |
Inherited from SupportsAnnotations
(via _DDLCompiles
, ClauseElement
, SupportsWrappingAnnotations
):
Property | _annotations_cache_key |
Undocumented |
Inherited from MemoizedHasCacheKey
(via _DDLCompiles
, ClauseElement
):
Method | _generate_cache_key |
return a cache key. |
Inherited from HasCacheKey
(via _DDLCompiles
, ClauseElement
, MemoizedHasCacheKey
):
Class Variable | inherit_cache |
Indicate if this .HasCacheKey instance should make use of the cache key generation scheme used by its immediate superclass. |
Class Method | _generate_cache_attrs |
generate cache key dispatcher for a new class. |
Class Method | _generate_cache_key_for_object |
Undocumented |
Method | _gen_cache_key |
return an optional cache key. |
Class Variable | __slots__ |
Undocumented |
Class Variable | _is_has_cache_key |
Undocumented |
Inherited from HasCopyInternals
(via _DDLCompiles
, ClauseElement
):
Method | _copy_internals |
Reassign internal elements to be clones of themselves. |
Inherited from Traversible
(via _DDLCompiles
, ClauseElement
):
Method | get_children |
Return immediate child .visitors.Traversible elements of this .visitors.Traversible . |
Method | __class_getitem__ |
Undocumented |