我们从Python开源项目中，提取了以下14个代码示例，用于说明如何使用django.db.models.expressions.Ref()。
def resolve_ref(self, name, allow_joins=True, reuse=None, summarize=False): if not allow_joins and LOOKUP_SEP in name: raise FieldError("Joined field references are not permitted in this query") if name in self.annotations: if summarize: # Summarize currently means we are doing an aggregate() query # which is executed as a wrapped subquery if any of the # aggregate() elements reference an existing annotation. In # that case we need to return a Ref to the subquery's annotation. return Ref(name, self.annotation_select[name]) else: return self.annotation_select[name] else: field_list = name.split(LOOKUP_SEP) field, sources, opts, join_list, path = self.setup_joins( field_list, self.get_meta(), self.get_initial_alias(), reuse) targets, _, join_list = self.trim_joins(sources, join_list, path) if len(targets) > 1: raise FieldError("Referencing multicolumn fields with F() objects " "isn't supported") if reuse is not None: reuse.update(join_list) col = targets[0].get_col(join_list[-1], sources[0]) return col
def rewrite_cols(self, annotation, col_cnt): # We must make sure the inner query has the referred columns in it. # If we are aggregating over an annotation, then Django uses Ref() # instances to note this. However, if we are annotating over a column # of a related model, then it might be that column isn't part of the # SELECT clause of the inner query, and we must manually make sure # the column is selected. An example case is: # .aggregate(Sum('author__awards')) # Resolving this expression results in a join to author, but there # is no guarantee the awards column of author is in the select clause # of the query. Thus we must manually add the column to the inner # query. orig_exprs = annotation.get_source_expressions() new_exprs = [] for expr in orig_exprs: if isinstance(expr, Ref): # Its already a Ref to subquery (see resolve_ref() for # details) new_exprs.append(expr) elif isinstance(expr, Col): # Reference to column. Make sure the referenced column # is selected. col_cnt += 1 col_alias = '__col%d' % col_cnt self.annotations[col_alias] = expr self.append_annotation_mask([col_alias]) new_exprs.append(Ref(col_alias, expr)) else: # Some other expression not referencing database values # directly. Its subexpression might contain Cols. new_expr, col_cnt = self.rewrite_cols(expr, col_cnt) new_exprs.append(new_expr) annotation.set_source_expressions(new_exprs) return annotation, col_cnt
def rewrite_cols(self, annotation, col_cnt): # We must make sure the inner query has the referred columns in it. # If we are aggregating over an annotation, then Django uses Ref() # instances to note this. However, if we are annotating over a column # of a related model, then it might be that column isn't part of the # SELECT clause of the inner query, and we must manually make sure # the column is selected. An example case is: # .aggregate(Sum('author__awards')) # Resolving this expression results in a join to author, but there # is no guarantee the awards column of author is in the select clause # of the query. Thus we must manually add the column to the inner # query. orig_exprs = annotation.get_source_expressions() new_exprs = [] for expr in orig_exprs: # FIXME: These conditions are fairly arbitrary. Identify a better # method of having expressions decide which code path they should # take. if isinstance(expr, Ref): # Its already a Ref to subquery (see resolve_ref() for # details) new_exprs.append(expr) elif isinstance(expr, (WhereNode, Lookup)): # Decompose the subexpressions further. The code here is # copied from the else clause, but this condition must appear # before the contains_aggregate/is_summary condition below. new_expr, col_cnt = self.rewrite_cols(expr, col_cnt) new_exprs.append(new_expr) elif isinstance(expr, Col) or (expr.contains_aggregate and not expr.is_summary): # Reference to column. Make sure the referenced column # is selected. col_cnt += 1 col_alias = '__col%d' % col_cnt self.annotations[col_alias] = expr self.append_annotation_mask([col_alias]) new_exprs.append(Ref(col_alias, expr)) else: # Some other expression not referencing database values # directly. Its subexpression might contain Cols. new_expr, col_cnt = self.rewrite_cols(expr, col_cnt) new_exprs.append(new_expr) annotation.set_source_expressions(new_exprs) return annotation, col_cnt
