What is the legal test applicable to a motion to disqualify a previous attorney from acting as counsel to a subsequent client?

The legal standard applicable to appellant's motion, is the substantial relationship test: "Where the potential conflict is one that arises from the successive representation of clients with potentially adverse interests, the courts have recognized that the chief fiduciary value jeopardized is that of client confidentiality. Thus, where a former client seeks to have a previous attorney disqualified from serving as counsel to a successive client in litigation adverse to the interests of the first client, the governing test requires that the client demonstrate a 'substantial relationship' between the subjects of the antecedent and current representation.' " (Flatt v. Superior Court (1994) 9 Cal.4th 275, 283, original italics.) In this case there can be no dispute that a substantial relationship exists between the alleged representation of appellant, and the subsequent representation of respondent, because they concern the identical subject matter, i.e., the question whether and under what circumstances appellant may obtain custody of, or maintain her relationship with the minor child. (See In re Marriage of Zimmerman (1993) 16 Cal.App.4th 556, 563 [no dispute exists on issue of "substantial relationship" when former client consults an attorney on the identical subject matter of the subsequent representation].)